feat(chat): add transient presence flash and 5s reconnect grace period

docs/STREAMING.md

@@ -68,13 +68,15 @@ Baseline packaging:
 | ---------------- | ------------------------------------------------------------------------------------- |
 | Protocol         | Standard live HLS, not LL-HLS                                                         |
 | Segment format   | fMP4 (`.m4s`) with an initialization segment; `.ts` may be supported during migration |
-| Segment target   | 6 seconds (`HLS_SEGMENT_DURATION=6`)                                                  |
-| Live window      | 24 segments (`HLS_WINDOW_SIZE=24`), about 144 seconds                                 |
+| Segment target   | 2 seconds (`HLS_SEGMENT_DURATION=2`)                                                  |
+| Live window      | 12 segments (`HLS_WINDOW_SIZE=12`), about 24 seconds                                  |
 | Codec path       | Stream copy for conforming assets; no per-viewer transcoding                          |
 | Expected latency | 15–35 seconds                                                                         |
 
 Segment boundaries must align with keyframes. Timestamps must remain monotonic across item boundaries. If FFmpeg cannot preserve a seamless timeline, insert `#EXT-X-DISCONTINUITY` and publish the corresponding new initialization segment.
 
+**Segment duration rationale**: 2s segments were chosen to lower end-to-end latency and keep the live window small (12 × 2s = 24s) while staying well above the manifest TTL (1–2s). The 2s GOP (30fps, `-g 60 -keyint_min 60`) already matches the target, so segments cut cleanly on keyframes. Longer durations (4–6s) increase latency and window depth; shorter than 2s inflates segment/request overhead and R2 Class B operations with little latency gain.
+
 Every segment name is globally unique and never reused, including after a restart or playlist loop. Use a run/session ID plus a monotonically increasing media sequence, for example:
 
 ```text
@@ -128,6 +130,8 @@ The manifest must refresh within a few seconds. Repeated segment requests should
 
 The SPA uses native HLS where supported and hls.js elsewhere.
 
+- Firefox has no native HLS, so it uses the hls.js/MSE path; fMP4 (`.m4s` + init segment) is supported. Autoplay requires `muted` + `playsinline`, which are already set on the `<video>` element.
+
 - Start at the live edge with a conservative buffer; do not seek to playlist start.
 - Treat the manifest as the synchronization source and target roughly three segments behind the newest segment unless testing supports a smaller margin.
 - Recover transient network/media errors with bounded retries and exponential backoff.
@@ -171,8 +175,8 @@ A release is streaming-ready only when one full playlist boundary has played wit
 | Variable                                   | Purpose                                            |
 | ------------------------------------------ | -------------------------------------------------- |
 | `PLAYLIST_PATH`                            | Active playlist file                               |
-| `HLS_SEGMENT_DURATION`                     | Target segment duration; baseline `6`              |
-| `HLS_WINDOW_SIZE`                          | Manifest window; baseline `24`                     |
+| `HLS_SEGMENT_DURATION`                     | Target segment duration; baseline `2`              |
+| `HLS_WINDOW_SIZE`                          | Manifest window; baseline `12`                     |
 | `R2_ENDPOINT`, `R2_REGION`                 | S3-compatible publication endpoint                 |
 | `R2_BUCKET_NAME`                           | Media bucket                                       |
 | `R2_ACCESS_KEY_ID`, `R2_SECRET_ACCESS_KEY` | Write credentials, available only to the publisher |