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).

package.json

@@ -10,8 +10,10 @@
     "test": "vitest run"
   },
   "dependencies": {
+    "jszip": "^3.10.1",
     "rrweb": "^2.0.0-alpha.4",
-    "jszip": "^3.10.1"
+    "ts-ebml": "^3.0.2",
+    "webm-muxer": "^5.1.4"
   },
   "devDependencies": {
     "@crxjs/vite-plugin": "^2.0.0-beta.25",
@@ -20,4 +22,4 @@
     "vite": "^5.4.2",
     "vitest": "^4"
   }
-}
+}