diff --git a/tests/background/webm-remux.test.ts b/tests/background/webm-remux.test.ts
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+// tests/background/webm-remux.test.ts
+//
+// Plan 01-08 Task 2: RED unit-level contract tests for the new
+// `remuxSegments()` helper that will live in
+// `src/background/webm-remux.ts` (created in Task 3).
+//
+// Pins the five invariants that the remux pipeline must honor in order
+// for SPEC §10 #7 (`last_30sec.webm plays back in a browser`) to hold:
+//
+//   1. Single-EBML-header output. The bytes returned by remuxSegments
+//      contain EXACTLY ONE `1A 45 DF A3` (EBML) and EXACTLY ONE
+//      `18 53 80 67` (Segment). That's the defining structural
+//      property the D-14-remux amendment commits to.
+//
+//   2. Size sanity. The output blob's byte count lives within
+//      [0.7×, 1.3×] of the summed input sizes — guards against
+//      silently dropping content OR ballooning it.
+//
+//   3. ffprobe-reported `format=duration` >= 25 s. Mirrors the gate the
+//      operator-facing test in tests/offscreen/webm-playback.test.ts
+//      enforces but at the unit-helper level.
+//
+//   4. ffprobe `-count_frames` reports `[905, 912]` frames inclusive
+//      (the per-segment sum is 301+300+311=912 frames per the d13
+//      byte-level probe; the muxer may legitimately drop at most one
+//      partial frame per segment boundary → 3 boundaries × 1 frame =
+//      ±3 absolute frames tolerance). I-01 fix during plan checker
+//      pass: tightened from the prior ±20% band to ±1% / ±3 frames.
+//
+//   5. Empty input ⇒ empty Blob. `remuxSegments([])` returns a
+//      `Promise<Blob>` whose `.size === 0`. Defense-in-depth — the
+//      saveArchive EmptyVideoBufferError throw guards this path
+//      upstream, but the helper must be safe on its own.
+//
+// The fixture used here is `tests/fixtures/last_30sec.webm` sliced at
+// the byte offsets the d13 debug session documented (0 / 509038 /
+// 970967 — verified empirically against the committed fixture at the
+// top of this test file's authoring session).
+//
+// Skip discipline: ffprobe-dependent tests gate behind `ffprobeAvailable()`
+// matching the pattern in tests/offscreen/webm-playback.test.ts. The
+// byte-magic tests (1, 2, 5) do NOT skip — they run anywhere.
+//
+// This file is RED at land time (the `src/background/webm-remux.ts`
+// module does not yet exist). Task 3 implements remuxSegments and
+// flips all 5 to GREEN.
+
+import { describe, it, expect } from 'vitest';
+import { readFileSync, existsSync, statSync, writeFileSync, mkdtempSync, rmSync } from 'node:fs';
+import { spawnSync } from 'node:child_process';
+import { fileURLToPath } from 'node:url';
+import { dirname, resolve, join } from 'node:path';
+import { tmpdir } from 'node:os';
+
+import type { VideoSegment } from '../../src/shared/types';
+
+const here = dirname(fileURLToPath(import.meta.url));
+const FIXTURE_PATH = resolve(here, '..', '..', 'tests', 'fixtures', 'last_30sec.webm');
+const FFPROBE_BIN = '/usr/bin/ffprobe';
+
+// Byte offsets of each EBML header in `tests/fixtures/last_30sec.webm`,
+// per the d13 debug session probe (Evidence/H4 byte-level EBML probe).
+// Verified at Plan 01-08 Task 2 land time against the committed
+// fixture: offsets [0, 509038, 970967]; total 1_633_459 bytes.
+const SEG1_START = 0;
+const SEG2_START = 509038;
+const SEG3_START = 970967;
+
+// Single-EBML invariant constants.
+const EBML_MAGIC = new Uint8Array([0x1a, 0x45, 0xdf, 0xa3]);
+const SEGMENT_MAGIC = new Uint8Array([0x18, 0x53, 0x80, 0x67]);
+
+// Per-segment frame counts from the d13 byte-level probe (segments
+// individually counted via ffprobe -count_frames):
+//   seg1 = 301, seg2 = 300, seg3 = 311 → sum = 912.
+// I-01 tolerance: muxer may drop at most one partial frame per segment
+// boundary (3 boundaries → ±3 absolute frames).
+const EXPECTED_FRAME_COUNT_LOWER = 905;
+const EXPECTED_FRAME_COUNT_UPPER = 912;
+
+// Playable-duration floor — mirrors MIN_PLAYABLE_DURATION_MS in
+// tests/offscreen/webm-playback.test.ts. The recorder lifecycle holds
+// 3 × 10 s segments minus boundary slack; gate at 25 s.
+const MIN_DURATION_MS = 25_000;
+
+// Size sanity band (input-sum × [lower, upper]). Catches silent content
+// drop AND container-overhead explosion.
+const SIZE_LOWER_RATIO = 0.7;
+const SIZE_UPPER_RATIO = 1.3;
+
+/**
+ * Predicate gating tests that shell out to ffprobe. Same shape as the
+ * helper in tests/offscreen/webm-playback.test.ts (kept inline rather
+ * than imported because the two test files live in distinct
+ * directories and Vitest discourages cross-tree relative test
+ * imports). Mirror updates between the two by hand.
+ *
+ * @returns true if /usr/bin/ffprobe exists and is a regular file.
+ */
+function ffprobeAvailable(): boolean {
+  try {
+    return existsSync(FFPROBE_BIN) && statSync(FFPROBE_BIN).isFile();
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * Count non-overlapping byte-window matches of `magic` inside `bytes`.
+ * Pure 4-byte pattern scan; no false-positive risk at the fixture
+ * scale (~1.6 MB) — EBML element IDs are structurally rare enough that
+ * a naive scan is correct, but more importantly the assertion uses
+ * EXACT-1 counts so any spurious match is a real bug surface.
+ *
+ * @param bytes - Byte array to scan.
+ * @param magic - 4-byte EBML element identifier.
+ * @returns Count of occurrences.
+ */
+function countMagic(bytes: Uint8Array, magic: Uint8Array): number {
+  if (magic.length !== 4) {
+    throw new Error(`countMagic expects 4-byte magic; got ${magic.length}`);
+  }
+  let count = 0;
+  for (let i = 0; i + 4 <= bytes.length; i++) {
+    if (
+      bytes[i] === magic[0] &&
+      bytes[i + 1] === magic[1] &&
+      bytes[i + 2] === magic[2] &&
+      bytes[i + 3] === magic[3]
+    ) {
+      count++;
+    }
+  }
+  return count;
+}
+
+/**
+ * Slice the canonical 3-segment fixture into 3 standalone WebM Blobs
+ * at the byte offsets the d13 byte-level probe identified. Each
+ * returned Blob is a complete, individually playable ~10 s WebM with
+ * its own EBML header + Segment + Cluster tree.
+ *
+ * The `timestamp` field on each VideoSegment uses the synthetic
+ * monotonic sequence `1_000_000 + i*10_000` (microsecond-style
+ * placeholders matching the recorder-side rotation cadence). The
+ * remux helper sorts by timestamp ascending so the relative ordering
+ * is what matters, not the absolute value.
+ *
+ * @returns Three VideoSegment entries, one per fixture segment.
+ */
+function splitFixtureIntoSegments(): VideoSegment[] {
+  const buf = readFileSync(FIXTURE_PATH);
+  const total = buf.length;
+  const seg1 = buf.subarray(SEG1_START, SEG2_START);
+  const seg2 = buf.subarray(SEG2_START, SEG3_START);
+  const seg3 = buf.subarray(SEG3_START, total);
+  return [
+    { data: new Blob([new Uint8Array(seg1)], { type: 'video/webm' }), timestamp: 1_000_000 },
+    { data: new Blob([new Uint8Array(seg2)], { type: 'video/webm' }), timestamp: 1_010_000 },
+    { data: new Blob([new Uint8Array(seg3)], { type: 'video/webm' }), timestamp: 1_020_000 },
+  ];
+}
+
+/**
+ * Read container-level format duration via ffprobe. Mirrors the
+ * helper in webm-playback.test.ts and returns NaN on parse failure.
+ *
+ * @param fixturePath - Absolute path to the WebM file.
+ * @returns Duration in milliseconds, or NaN on parse failure.
+ */
+function probeContainerDurationMs(fixturePath: string): number {
+  const proc = spawnSync(
+    FFPROBE_BIN,
+    [
+      '-v', 'error',
+      '-show_entries', 'format=duration',
+      '-of', 'default=noprint_wrappers=1:nokey=1',
+      '-i', fixturePath,
+    ],
+    {
+      stdio: ['ignore', 'pipe', 'pipe'],
+      encoding: 'utf-8',
+      timeout: 30_000,
+      maxBuffer: 1 * 1024 * 1024,
+    },
+  );
+  if (proc.signal !== null) {
+    throw new Error(`ffprobe killed by signal ${proc.signal}`);
+  }
+  const stdout = (proc.stdout ?? '').trim();
+  const seconds = parseFloat(stdout);
+  return Number.isFinite(seconds) ? Math.round(seconds * 1000) : Number.NaN;
+}
+
+/**
+ * Read decoded frame count via `ffprobe -count_frames`. Counts the
+ * video stream's nb_read_frames property (the actual decode-side
+ * count, not the container-claimed count which can lie under the
+ * D-13 multi-EBML pathology).
+ *
+ * @param fixturePath - Absolute path to the WebM file.
+ * @returns Frame count, or NaN on parse failure.
+ */
+function probeDecodedFrameCount(fixturePath: string): number {
+  const proc = spawnSync(
+    FFPROBE_BIN,
+    [
+      '-v', 'error',
+      '-count_frames',
+      '-select_streams', 'v:0',
+      '-show_entries', 'stream=nb_read_frames',
+      '-of', 'default=noprint_wrappers=1:nokey=1',
+      '-i', fixturePath,
+    ],
+    {
+      stdio: ['ignore', 'pipe', 'pipe'],
+      encoding: 'utf-8',
+      timeout: 30_000,
+      maxBuffer: 1 * 1024 * 1024,
+    },
+  );
+  if (proc.signal !== null) {
+    throw new Error(`ffprobe killed by signal ${proc.signal}`);
+  }
+  const stdout = (proc.stdout ?? '').trim();
+  const n = parseInt(stdout, 10);
+  return Number.isFinite(n) ? n : Number.NaN;
+}
+
+/**
+ * Write a Blob to a fresh tmpfile and return the path. Caller is
+ * responsible for cleanup via the returned dir handle.
+ *
+ * @param blob - Blob to materialize on disk.
+ * @returns { path, dir } — `dir` to remove recursively after use.
+ */
+async function blobToTmpFile(blob: Blob): Promise<{ path: string; dir: string }> {
+  const dir = mkdtempSync(join(tmpdir(), 'webm-remux-test-'));
+  const path = join(dir, 'remuxed.webm');
+  const ab = await blob.arrayBuffer();
+  writeFileSync(path, Buffer.from(ab));
+  return { path, dir };
+}
+
+describe('remuxSegments (Plan 01-08 Task 2 — RED until Task 3)', () => {
+  it('emits exactly one EBML header and one Segment element', async () => {
+    // Dynamic import wrapped in try/catch so RED before Task 3 lands
+    // produces a precise message rather than a module-resolution
+    // crash that hides the contract.
+    let remux: typeof import('../../src/background/webm-remux');
+    try {
+      remux = await import('../../src/background/webm-remux');
+    } catch (e) {
+      expect.fail(
+        `src/background/webm-remux.ts does not exist yet — this is the Task 3 GREEN gate. Error: ${String(e)}`,
+      );
+    }
+    const segments = splitFixtureIntoSegments();
+    const out = await remux.remuxSegments(segments);
+    const bytes = new Uint8Array(await out.arrayBuffer());
+
+    const ebmlCount = countMagic(bytes, EBML_MAGIC);
+    const segmentCount = countMagic(bytes, SEGMENT_MAGIC);
+    expect(
+      ebmlCount,
+      `Expected exactly 1 EBML header magic (0x1A 0x45 0xDF 0xA3) in remux output, found ${ebmlCount}. ` +
+        `The whole point of D-14-remux is to collapse 3 source EBML headers into 1.`,
+    ).toBe(1);
+    expect(
+      segmentCount,
+      `Expected exactly 1 Segment element magic (0x18 0x53 0x80 0x67) in remux output, found ${segmentCount}. ` +
+        `Multiple Segment elements mean the merge collapsed into a multi-segment Matroska (broken under SPEC §10 #7).`,
+    ).toBe(1);
+  });
+
+  it('emits a blob whose size is within [0.7x, 1.3x] of the input total', async () => {
+    let remux: typeof import('../../src/background/webm-remux');
+    try {
+      remux = await import('../../src/background/webm-remux');
+    } catch (e) {
+      expect.fail(`webm-remux module missing: ${String(e)}`);
+    }
+    const segments = splitFixtureIntoSegments();
+    const inputTotal = segments.reduce((sum, seg) => sum + seg.data.size, 0);
+    const out = await remux.remuxSegments(segments);
+    const lower = inputTotal * SIZE_LOWER_RATIO;
+    const upper = inputTotal * SIZE_UPPER_RATIO;
+    expect(
+      out.size,
+      `Output size ${out.size} bytes; input sum ${inputTotal}; expected band ` +
+        `[${lower.toFixed(0)}, ${upper.toFixed(0)}]. Below band = silent content drop; ` +
+        `above band = container overhead explosion (re-encode regression?).`,
+    ).toBeGreaterThanOrEqual(lower);
+    expect(out.size).toBeLessThanOrEqual(upper);
+  });
+
+  it.skipIf(!ffprobeAvailable())(
+    'ffprobe format=duration on the remuxed output is at least 25 s',
+    async () => {
+      let remux: typeof import('../../src/background/webm-remux');
+      try {
+        remux = await import('../../src/background/webm-remux');
+      } catch (e) {
+        expect.fail(`webm-remux module missing: ${String(e)}`);
+      }
+      const segments = splitFixtureIntoSegments();
+      const out = await remux.remuxSegments(segments);
+      const handle = await blobToTmpFile(out);
+      try {
+        const durationMs = probeContainerDurationMs(handle.path);
+        expect(
+          durationMs,
+          `ffprobe reported container duration=${durationMs} ms for remux output. ` +
+            `SPEC §10 #7 floor is ${MIN_DURATION_MS} ms. ` +
+            `If <10 s, the multi-EBML pathology returned (Plan 01-08 regression). ` +
+            `If 0/NaN, the muxer produced no Info.Duration EBML (check Muxer config).`,
+        ).toBeGreaterThanOrEqual(MIN_DURATION_MS);
+      } finally {
+        rmSync(handle.dir, { recursive: true, force: true });
+      }
+    },
+  );
+
+  it.skipIf(!ffprobeAvailable())(
+    'ffprobe -count_frames reports between 905 and 912 frames inclusive',
+    async () => {
+      let remux: typeof import('../../src/background/webm-remux');
+      try {
+        remux = await import('../../src/background/webm-remux');
+      } catch (e) {
+        expect.fail(`webm-remux module missing: ${String(e)}`);
+      }
+      const segments = splitFixtureIntoSegments();
+      const out = await remux.remuxSegments(segments);
+      const handle = await blobToTmpFile(out);
+      try {
+        const frames = probeDecodedFrameCount(handle.path);
+        expect(
+          frames,
+          `ffprobe -count_frames reported ${frames} frames in remux output. ` +
+            `Expected band [${EXPECTED_FRAME_COUNT_LOWER}, ${EXPECTED_FRAME_COUNT_UPPER}] ` +
+            `(per-segment sum 301+300+311=912 ± 3 boundary partial-frame drops). ` +
+            `Below band = frame loss (silent SimpleBlock-skip bug); above band = ` +
+            `duplicate-frame bug (re-emitting the same SimpleBlock from multiple sources).`,
+        ).toBeGreaterThanOrEqual(EXPECTED_FRAME_COUNT_LOWER);
+        expect(frames).toBeLessThanOrEqual(EXPECTED_FRAME_COUNT_UPPER);
+      } finally {
+        rmSync(handle.dir, { recursive: true, force: true });
+      }
+    },
+  );
+
+  it('empty input returns an empty Blob', async () => {
+    let remux: typeof import('../../src/background/webm-remux');
+    try {
+      remux = await import('../../src/background/webm-remux');
+    } catch (e) {
+      expect.fail(`webm-remux module missing: ${String(e)}`);
+    }
+    const out = await remux.remuxSegments([]);
+    expect(out).toBeInstanceOf(Blob);
+    expect(
+      out.size,
+      'Empty input must produce an empty blob — saveArchive guards this path upstream but ' +
+        'remuxSegments must be safe on its own (defense-in-depth, T-1-08-01 + T-1-08-02).',
+    ).toBe(0);
+  });
+});