test+docs: packet ring stress test + Frigate dual-input guide (v0.2 Step 6)

Тесты:
- libcuframes/tests/test_packet_ring.c — 2 scenarios:
  1) normal flow: 1 pub × 1 sub × 2000 packets, varied sizes, GOP=30,
     payload integrity check (seq в первых 8 байтах + pattern). PTS
     monotonicity, first KEY seq, нет data errors.
  2) slow consumer (10ms delay): publisher 200 fps, subscriber должен
     detect OVERRUN, library resync на keyframe — verify received >10
     даже на сильно медленном консьюмере.
- libcuframes/tests/CMakeLists.txt: add_test packet_ring_basic.

Docs:
- CHANGELOG.md: новая [Unreleased] секция с full v0.2 highlights и
  явно declared limitations (sub-stream, audio, codec change → v0.3).
- docs/integrations/frigate.md: новая секция "v0.2: dual-input (detect +
  record через один RTSP)" с config example, requirements, trade-offs.

Связано: #2, PR #4. Step 6 (final) перед снятием draft.

CHANGELOG.md

@@ -5,6 +5,45 @@
 Формат основан на [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 проект следует [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased] — v0.2 work in progress
+
+См. PR [#4](https://git.goldix.org/gx/cuframes/pulls/4).
+
+### Added
+
+- **Encoded packet ring** — параллельный ring для H.264/H.265 NAL units
+  (отдельный SHM `/dev/shm/cuframes-<key>-packets`, variable-length byte
+  buffer + slot index, seqlock-style read для защиты от overrun).
+- **Wire protocol v2** (`proto_version = 2` в SHM header). Backward-compat:
+  v2 publishers принимают v1 subscribers (frames-only).
+- **Public C API** (`include/cuframes/cuframes.h`):
+  - `cuframes_publisher_enable_packets(opts)` — активирует ring
+  - `cuframes_publisher_set_codec_extradata(data, size)` — SPS/PPS
+  - `cuframes_publisher_publish_packet(data, size, pts, dts, flags)`
+  - `cuframes_subscriber_enable_packets()` + `_next_packet()` + accessors
+  - `cuframes_subscriber_get_codec_params(codec_id, extradata, size)`
+- **`cuframes::Publisher`** (C++ RAII): `enable_packets`, `set_codec_extradata`,
+  `publish_packet` методы.
+- **`cuframes-rtsp-source`**: новый CLI flag `--enable-packet-ring`.
+  Дублирует `AVPacket` в encoded ring до передачи декодеру.
+- **FFmpeg demuxer `cuframes_packets://<key>`** (отдельная ветка
+  [gx/ffmpeg-patched PR #1](https://git.goldix.org/gx/ffmpeg-patched/pulls/1)).
+  Companion к `cuframes://`. Use case: Frigate `record` role без
+  второго RTSP к камере.
+- **4 новых error codes**: `PACKET_OVERSIZED`, `NO_PACKET_RING`,
+  `NO_CODEC_PARAMS`, `PACKET_OVERRUN`.
+- **Stress test** `libcuframes/tests/test_packet_ring.c`: 2 scenarios —
+  normal flow (1 pub × 1 sub × 2000 packets, integrity check) +
+  slow consumer (must hit OVERRUN + library auto-resync на keyframe).
+- **Protocol spec §10** в `docs/protocol.md` (397 строк): byte-exact
+  layout, seqlock semantics, late-subscriber GOP-aligned start.
+
+### Limitations (документировано)
+
+- Sub-stream selection отложено в v0.3 (`<key>-substream-<N>` naming).
+- Audio packets — v0.3 (тот же ring layout, codec_id = audio).
+- Codec change mid-stream — требует publisher destroy+recreate.
+
 ## [0.1.0] — 2026-05-17
 
 Первый функциональный release с production deployment.