feat(fix-d12): add TransferredVideoChunk wire-format type in src/shared/types.ts

- Add TransferredVideoChunk { data: string (base64); type: string
  (MIME); timestamp: number; isFirst?: boolean } as the
  JSON-serializable shape for chunks traversing the offscreen↔SW
  chrome.runtime.Port boundary.
- Retarget PortMessage.chunks?: TransferredVideoChunk[] (was
  VideoChunk[]). The in-memory VideoChunk type is unchanged — both
  the offscreen ring buffer and the SW-side merge step keep using
  it after decoding the wire payload.
- No behavior change yet; this commit only lands the type. The
  encode/decode call sites land in the next two commits.

Refs: debug session d12-blob-port-transfer-fails.

src/shared/types.ts

@@ -29,16 +29,33 @@ export type PortMessageType =
 
 export interface PortMessage {
   type: PortMessageType;
-  chunks?: VideoChunk[];
+  // Wire-format (post-D-12 fix): chunks travel as TransferredVideoChunk[]
+  // because chrome.runtime.Port JSON-serializes payloads, and
+  // JSON.stringify(blob) === "{}" loses binary content. The receive
+  // side reconstructs VideoChunk[] via src/shared/binary.ts.
+  chunks?: TransferredVideoChunk[];
 }
 
-// Видеобуфер
+// In-memory chunk shape used by the offscreen ring buffer and by
+// mergeVideoChunks after the SW decodes the wire format.
 export interface VideoChunk {
   data: Blob;
   timestamp: number;
   isFirst?: boolean;
 }
 
+// Wire-format for video chunks traveling across the offscreen↔SW
+// chrome.runtime.Port boundary. Replaces the previous VideoChunk[]
+// payload, which failed because Blob is not JSON-serializable.
+// See debug session d12-blob-port-transfer-fails and the GREEN block
+// of tests/offscreen/port-serialization.test.ts for the pinned shape.
+export interface TransferredVideoChunk {
+  data: string;       // base64-encoded blob bytes (no data: prefix)
+  type: string;       // MIME type to apply when reconstructing the Blob
+  timestamp: number;
+  isFirst?: boolean;
+}
+
 // Лог событий пользователя
 export interface UserEvent {
   timestamp: number;