feat(01-08): install ts-ebml + webm-muxer; pin SW-compat via deps test

- Add ts-ebml ^3.0.2 (parse half) and webm-muxer ^5.1.4 (write half) per
  CONTEXT.md amendment D-14-remux; both MIT, both verified SW-compatible
  in the d13 debug-session library survey.
- tests/background/webm-remux-deps.test.ts pins two contracts:
  (a) named exports surface (Muxer + ArrayBufferTarget + Decoder).
  (b) both libraries import cleanly when window/document are absent on
      globalThis — guards the published dist against accidentally
      acquiring DOM globals on the hot path that would crash the
      Chrome service-worker runtime.
- Note: webm-muxer 5.1.4 upstream-deprecated in favor of Mediabunny; the
  pinned version still meets the d13 architectural requirement
  (single-EBML output via addVideoChunkRaw). Migration to Mediabunny is
  out of scope for Plan 01-08 and would require a new ADR.
- Baseline 53 GREEN + 2 new GREEN; tsc clean; 2 webm-playback duration
  RED still pending (drive to GREEN in Tasks 3-5).

tests/background/webm-remux-deps.test.ts

@@ -0,0 +1,137 @@
+// tests/background/webm-remux-deps.test.ts
+//
+// Plan 01-08 Task 1: SW-compatibility + presence contract for the two new
+// runtime dependencies that the WebM remux pipeline rests on. Pins the
+// architectural commitment that landed in CONTEXT.md amendment D-14-remux:
+//   - `ts-ebml`  ^3.0.2 (MIT, parses each VideoSegment's EBML structure)
+//   - `webm-muxer` ^5.1.4 (MIT, writes the single-EBML-headered output)
+//
+// Both libraries were surveyed in `.planning/debug/d13-multi-ebml-concat-
+// unplayable.md` (Evidence/library-survey, lines 380-410). They are pure
+// JS, pure ESM/CJS, and were grep-verified at survey time to contain no
+// hard DOM-global references on the hot path. Chrome's service-worker
+// runtime (where `remuxSegments()` will execute) does not provide
+// `window` or `document`; this test pins that compat at the
+// devDependency-import surface so a future bump that accidentally adds
+// a DOM global is caught at test time rather than at runtime in a
+// production SW.
+//
+// Test 1 asserts named-export presence (RED until `npm install` lands).
+// Test 2 asserts both libraries load under default Node globals without
+// referencing `window` or `document` synchronously at import time.
+//
+// Skip discipline: none — these are pure import-shape tests, no external
+// binaries, no fixtures. Vitest's default Node environment is sufficient.
+
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+
+interface GlobalSnapshot {
+  window: unknown;
+  document: unknown;
+  hadWindow: boolean;
+  hadDocument: boolean;
+}
+
+/**
+ * Capture the current values of `window` and `document` on `globalThis`
+ * so they can be restored after a test that deletes them. Vitest's
+ * `vi.stubGlobal` is not used here because we want to assert the
+ * absence of the globals at import time, not just stub them.
+ *
+ * @returns Snapshot the caller passes back to {@link restoreGlobals}.
+ */
+function snapshotGlobals(): GlobalSnapshot {
+  const g = globalThis as unknown as Record<string, unknown>;
+  return {
+    window: g.window,
+    document: g.document,
+    hadWindow: 'window' in g,
+    hadDocument: 'document' in g,
+  };
+}
+
+/**
+ * Restore the globals captured by {@link snapshotGlobals}. Idempotent.
+ *
+ * @param snap - Snapshot returned by {@link snapshotGlobals}.
+ */
+function restoreGlobals(snap: GlobalSnapshot): void {
+  const g = globalThis as unknown as Record<string, unknown>;
+  if (snap.hadWindow) {
+    g.window = snap.window;
+  } else {
+    delete g.window;
+  }
+  if (snap.hadDocument) {
+    g.document = snap.document;
+  } else {
+    delete g.document;
+  }
+}
+
+describe('webm-remux dependencies (Plan 01-08 Task 1)', () => {
+  it('exports Muxer + ArrayBufferTarget + Decoder', async () => {
+    // Dynamic import so a missing package surfaces as a precise test
+    // failure ("Cannot find module 'webm-muxer'") rather than a Vitest
+    // collection error that hides which dependency is the cause.
+    const webmMuxer = await import('webm-muxer');
+    expect(webmMuxer.Muxer).toBeDefined();
+    expect(webmMuxer.ArrayBufferTarget).toBeDefined();
+
+    const tsEbml = await import('ts-ebml');
+    expect(tsEbml.Decoder).toBeDefined();
+  });
+
+  describe('loads under default Node globals without DOM-global ReferenceErrors', () => {
+    let snap: GlobalSnapshot;
+
+    beforeEach(() => {
+      snap = snapshotGlobals();
+      const g = globalThis as unknown as Record<string, unknown>;
+      delete g.window;
+      delete g.document;
+    });
+
+    afterEach(() => {
+      restoreGlobals(snap);
+    });
+
+    it('webm-muxer + ts-ebml do not throw on import when window/document are absent', async () => {
+      // The Chrome service-worker runtime provides neither `window` nor
+      // `document`. If either library's published dist references one
+      // of these synchronously at module evaluation time (e.g. a UMD
+      // wrapper falling through to `window`), the import below would
+      // throw a ReferenceError and this test would fail with a clear
+      // signal.
+      //
+      // ts-ebml's UMD wrapper does contain a `typeof window` check
+      // with a `self`/`global` fallback per the d13 library survey
+      // — `typeof` does NOT throw on undeclared identifiers per
+      // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof
+      // so the fallback path resolves cleanly.
+      //
+      // webm-muxer is documented zero-DOM-ref.
+      let webmMuxerError: unknown = null;
+      try {
+        await import('webm-muxer');
+      } catch (e) {
+        webmMuxerError = e;
+      }
+      expect(
+        webmMuxerError,
+        `webm-muxer threw at import time without window/document: ${String(webmMuxerError)}`,
+      ).toBeNull();
+
+      let tsEbmlError: unknown = null;
+      try {
+        await import('ts-ebml');
+      } catch (e) {
+        tsEbmlError = e;
+      }
+      expect(
+        tsEbmlError,
+        `ts-ebml threw at import time without window/document: ${String(tsEbmlError)}`,
+      ).toBeNull();
+    });
+  });
+});