fix(01-review): WR-07 base64ToBlob empty-input shortcut + SW-side empty-segment filter

tests/offscreen/port-serialization.test.ts

@@ -193,3 +193,44 @@ describe('port serialization (GREEN — pins the eventual fix contract)', () =>
     expect(merged.size).not.toBe(75); // the bug is gone in the fixed form.
   });
 });
+
+describe('binary.ts adversarial input (WR-07 + sweep target #6)', () => {
+  // These tests exercise the PRODUCTION helpers in src/shared/binary.ts,
+  // not local re-implementations. They pin the WR-07 + sweep-#6
+  // defensive contracts:
+  //   1. Empty-string input → zero-byte Blob (early-return shortcut,
+  //      no atob('') round-trip).
+  //   2. Invalid base64 input → throws DOMException-like InvalidCharacterError,
+  //      caller is expected to wrap in try/catch.
+  //
+  // The empty-segment filter in SW's getVideoBufferFromOffscreen relies
+  // on (1) so the multi-EBML-header concat output never includes a stray
+  // zero-byte segment. The error-classification step relies on (2) being
+  // a regular throw so the per-segment try/catch can drop a single bad
+  // entry without aborting the whole transfer.
+
+  it('base64ToBlob("") returns a zero-byte Blob with the requested MIME', async () => {
+    const { base64ToBlob } = await import('../../src/shared/binary');
+    const blob = base64ToBlob('', 'video/webm');
+    expect(blob).toBeInstanceOf(Blob);
+    expect(blob.size).toBe(0);
+    expect(blob.type).toBe('video/webm');
+  });
+
+  it('blobToBase64(emptyBlob) round-trips through base64ToBlob to zero bytes', async () => {
+    const { blobToBase64, base64ToBlob } = await import('../../src/shared/binary');
+    const original = new Blob([], { type: 'video/webm' });
+    const b64 = await blobToBase64(original);
+    expect(b64).toBe('');
+    const restored = base64ToBlob(b64, 'video/webm');
+    expect(restored.size).toBe(0);
+  });
+
+  it('base64ToBlob throws on invalid base64 alphabet (caller wraps with try/catch)', async () => {
+    const { base64ToBlob } = await import('../../src/shared/binary');
+    // Characters outside the base64 alphabet — atob throws InvalidCharacterError
+    // synchronously. The SW caller (getVideoBufferFromOffscreen) has a
+    // try/catch that drops a single bad segment without aborting the batch.
+    expect(() => base64ToBlob('!!!not-base64!!!', 'video/webm')).toThrow();
+  });
+});