feat(fix-d12): decode chunks from base64 in SW BUFFER receive

- Read incoming port.chunks as TransferredVideoChunk[] (was
  VideoChunk[] — but that was a lie because Blob doesn't survive
  JSON serialization across the port boundary).
- Decode each wire chunk via base64ToBlob(wire.data, wire.type) and
  resolve VideoBufferResponse with the resulting VideoChunk[]. The
  existing mergeVideoChunks downstream sees real Blobs and produces
  a real WebM-prefixed merged blob.
- Defensive per-chunk decode: log + skip individual decode failures
  rather than blowing up the whole fetch. Falls back to
  video/webm;codecs=vp9 if the wire chunk somehow omits the type
  (defense-in-depth — the offscreen always populates it).
- Document the 2 s BUFFER_FETCH_TIMEOUT_MS budget: covers worst-case
  encode + post-message + JSON parse with > 1.5 s of headroom for
  the current 15-chunk × 100 KB sizing.

Refs: debug session d12-blob-port-transfer-fails, D-17 port lifecycle.

src/background/index.ts

@@ -1,12 +1,19 @@
 import { Logger } from '../shared/logger';
+import { base64ToBlob } from '../shared/binary';
 import type {
   Message,
+  TransferredVideoChunk,
   VideoChunk,
   SessionMetadata,
   VideoBufferResponse
 } from '../shared/types';
 import JSZip from 'jszip';
 
+// Default MIME applied when a wire chunk somehow lacks a type
+// field (defense-in-depth: in normal operation the offscreen recorder
+// always populates it from chunk.data.type). Matches D-20 strict codec.
+const VIDEO_MIME_FALLBACK = 'video/webm;codecs=vp9';
+
 const logger = new Logger('Main');
 
 // Состояние
@@ -88,6 +95,11 @@ chrome.runtime.onConnect.addListener((port) => {
   // per-request listener installed in getVideoBufferFromOffscreen).
 });
 
+// 2 s budget covers the worst-case round-trip: offscreen base64-encodes
+// up to ~15 chunks of ~100 KB each (~1.5 MB raw → ~2 MB base64) in
+// well under 100 ms, post-message + JSON parse adds < 50 ms, leaving
+// plenty of headroom. Bumping later is cheap if real-world recordings
+// produce significantly larger buffers; today this is sufficient.
 const BUFFER_FETCH_TIMEOUT_MS = 2_000;
 
 async function getVideoBufferFromOffscreen(): Promise<VideoBufferResponse> {
@@ -110,7 +122,30 @@ async function getVideoBufferFromOffscreen(): Promise<VideoBufferResponse> {
       ) {
         clearTimeout(timer);
         port.onMessage.removeListener(handler);
-        const chunks = (msg as { chunks?: VideoChunk[] }).chunks ?? [];
+        // D-12 fix: chunks arrive as TransferredVideoChunk[] (base64
+        // string + MIME). Decode each back into a VideoChunk so
+        // mergeVideoChunks keeps operating on real Blobs. See
+        // src/shared/binary.ts and the GREEN block of
+        // tests/offscreen/port-serialization.test.ts.
+        const wireChunks =
+          (msg as { chunks?: TransferredVideoChunk[] }).chunks ?? [];
+        const chunks: VideoChunk[] = [];
+        for (const wire of wireChunks) {
+          try {
+            chunks.push({
+              data: base64ToBlob(wire.data, wire.type || VIDEO_MIME_FALLBACK),
+              timestamp: wire.timestamp,
+              isFirst: wire.isFirst,
+            });
+          } catch (err) {
+            logger.warn(
+              'base64ToBlob failed; skipping chunk',
+              'timestamp:', wire.timestamp,
+              'isFirst:', wire.isFirst,
+              'error:', err,
+            );
+          }
+        }
         resolve({ chunks });
       }
     };