feat(fix-a3): rename TransferredVideoChunk → TransferredVideoSegment

Pure rename pass — zero behavioural change at any call site. The
prior "chunk" naming is a vestige of D-09..D-11's chunk-level
buffer; under D-13 the unit of transfer is a self-contained ~10 s
WebM segment, so the type name now matches the data shape.

Renames propagated atomically:
- src/shared/types.ts
  * TransferredVideoChunk → TransferredVideoSegment
    (also: the `isFirst?: boolean` field is dropped — D-13 segments
    are all implicitly their own header, so the flag is meaningless
    and only existed for the retired ring-buffer's pin semantics)
  * VideoChunk → VideoSegment (drops `isFirst?` for the same reason)
  * PortMessage.chunks? → PortMessage.segments?
  * VideoBufferResponse.chunks → VideoBufferResponse.segments
- src/offscreen/recorder.ts
  * import type rename
  * encodeAndSendBuffer's per-element accumulator + filter type
  * the port.postMessage payload field
- src/background/index.ts
  * import type rename
  * getVideoBufferFromOffscreen reads `(msg as {segments?}).segments`
    (matching the new wire field name)
  * empty-buffer returns `{ segments: [] }`
  * mergeVideoSegments signature takes VideoSegment[]
  * createArchive consumes videoBufferResponse.segments
  * saveArchive log line says "segments"

Verification:
- npx tsc --noEmit clean.
- npx vitest run → 28 passed / 2 failed (the 2 fixture-empirical
  ffmpeg dry-runs; unchanged — they're gated on ./smoke.sh regen).
- npm run build succeeds, all 60 modules transformed.
- grep predicates clean in src/:
  * no addChunk / trimAged / firstChunkSaved / isFirst
  * no TransferredVideoChunk / VideoChunk (old names)
  * 21 occurrences of new names propagated correctly
- Pre-existing port-serialization.test.ts still GREEN: its `isFirst`
  references are inside inline-test-scoped objects (not imports
  from production types), and its tests assert JSON-roundtrip
  behaviour rather than the production type shape.

src/shared/types.ts

@@ -21,7 +21,7 @@ export interface Message<T = any> {
   tabId?: number;
 }
 
-// Типы сообщений в long-lived port (offscreen ↔ SW; D-17 / Plan 04)
+// Типы сообщений в long-lived port (offscreen ↔ SW; D-17 / Plan 04 / D-13)
 export type PortMessageType =
   | 'PING'
   | 'REQUEST_BUFFER'
@@ -29,31 +29,35 @@ export type PortMessageType =
 
 export interface PortMessage {
   type: PortMessageType;
-  // 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[];
+  // Wire-format (D-12 base64 transfer + D-13 segment lifecycle):
+  // segments travel as TransferredVideoSegment[] because
+  // chrome.runtime.Port JSON-serializes payloads across extension
+  // contexts and JSON.stringify(blob) === "{}" loses binary content.
+  // Each entry is one self-contained ~10 s WebM segment (EBML header
+  // + seed keyframe). The receive side reconstructs VideoSegment[]
+  // via src/shared/binary.ts.
+  segments?: TransferredVideoSegment[];
 }
 
-// In-memory chunk shape used by the offscreen ring buffer and by
-// mergeVideoChunks after the SW decodes the wire format.
-export interface VideoChunk {
+// In-memory segment shape used by mergeVideoSegments after the SW
+// decodes the wire format. Под D-13 каждый сегмент — самодостаточный
+// WebM-блок ≈ 10 секунд (свой EBML-заголовок и стартовый keyframe).
+export interface VideoSegment {
   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 {
+// Wire-format for video segments traveling across the offscreen↔SW
+// chrome.runtime.Port boundary. Replaces the previous Blob payload,
+// which failed because Blob is not JSON-serializable.
+// See debug session d12-blob-port-transfer-fails (resolved) and the
+// GREEN block of tests/offscreen/port-serialization.test.ts for the
+// pinned shape. The header-pin flag from the D-09..D-11 era is gone
+// — under D-13 every segment IS implicitly its own header.
+export interface TransferredVideoSegment {
   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
@@ -86,9 +90,9 @@ export interface PopupState {
 
 // Ответы от Service Worker
 export interface VideoBufferResponse {
-  chunks: VideoChunk[];
+  segments: VideoSegment[];
 }
 
 export interface RrwebEventsResponse {
   events: any[];
-}
+}