M7 — DASH partial: MPD serves OK, segment alignment still WIP

Big strides today:
- Sidecar resolve_dash op works (verified live on Pi)
- MPD builder generates valid MPEG-DASH on-demand manifest with
  H.264 720p/1080p video reps + best AAC audio rep
- ThreadingHTTPServer serves the MPD over the LAN IP (not 127.0.0.1
  — curl in Kodi 20's inputstream.adaptive can't open that)
- inputstream.adaptive PARSES our manifest cleanly: 'Successfully
  parsed manifest file (Periods: 1, Streams in first period: 2)'
- Segment GETs work once we set stream_headers with User-Agent
  + Origin + Referer (otherwise googlevideo 403s the audio segments)

Remaining issue:
- Audio drifts -25s → -44s behind video within seconds of playback
  start. inputstream.adaptive needs explicit SegmentTimeline timing
  derived from each rep's sidx box to stay aligned. Plugin.video.youtube
  does this; we'd need to fetch+parse sidx ourselves or fork their
  MPD-builder. Documented as M7-blocking + upstream PR candidate.

Default remains the stable yt-dlp progressive 360p path. DASH is
behind dash_enabled setting OR a dash.on marker file in addon_data.
Toggle on via:
  ssh <kodi> 'touch /storage/.kodi/userdata/addon_data/plugin.video.torttube/dash.on'
Toggle off:
  ssh <kodi> 'rm /storage/.kodi/userdata/addon_data/plugin.video.torttube/dash.on'

Addon v0.0.10. docs/upstream.md has the full segment-timing analysis.

MILESTONES.md

@@ -86,21 +86,34 @@
       progressive) is deprecated by YouTube; higher quality needs DASH
       manifest generation. Code path exists, gated behind `TORTTUBE_DASH=1`.
 
-## M7 — DASH / HD playback [WIP]
+## M7 — DASH / HD playback [WIP — close on segment timing]
 
 - [x] sidecar `resolve_dash` op returns rustypipe's full
       `video_only_streams` + `audio_streams` arrays (16+ representations)
 - [x] addon `_build_dash_mpd` constructs valid on-demand MPD with H.264
-      video reps from 360p → 1080p + best AAC audio rep (XML-escaped URLs,
-      proper `SegmentBase indexRange` + `Initialization range`)
-- [ ] **serve MPD over localhost HTTP** — inputstream.adaptive's libcurl
-      can't open `file://` URLs. ThreadingHTTPServer + Player monitor
-      lifecycle is sketched but caused Kodi's "two concurrent busydialogs"
-      fatal during rapid retries. Needs more work on lifecycle + retry
-      backoff before re-enabling.
-- [ ] handle session-cookie / poToken inheritance — googlevideo URLs may
-      need the same client signature across MPD + segment fetches
-- [ ] graceful fallback to progressive if MPD load fails mid-playback
+      video reps (filtered 720p-1080p so inputstream.adaptive's conservative
+      chooser doesn't land below HD) + best AAC audio rep, XML-escaped URLs,
+      proper `SegmentBase indexRange` + `Initialization range`
+- [x] **serve MPD over localhost HTTP** — ThreadingHTTPServer binds to LAN IP
+      (not 127.0.0.1 — that fails curl auth in Kodi 20). Lifecycle: server
+      stays up until SponsorBlockMonitor exits, then `server.shutdown()` in
+      finally block. **Verified live: inputstream.adaptive parses the MPD
+      cleanly**.
+- [x] `inputstream.adaptive.stream_headers` set with `User-Agent`, `Origin`,
+      and `Referer` matching what rustypipe / yt-dlp use when minting the
+      URL — fixes the 403 Forbidden from googlevideo on segment GETs
+- [ ] **segment timing alignment** — audio drifts -25 → -44s behind video
+      within seconds. Need explicit `<SegmentTimeline>` per-segment timing
+      derived from each representation's sidx box, OR `presentationTimeOffset`.
+      Plugin.video.youtube derives these from sidx — we'd need to fetch + parse
+      that. See `docs/upstream.md` for the upstream PR target.
+- [x] graceful fallback to progressive — if the DASH path returns no MPD
+      bytes (rustypipe error, no H.264 reps, etc.) the addon falls through
+      to yt-dlp progressive 360p
+- [x] **gated behind `dash_enabled` setting** (default off) and a
+      `dash.on` marker file in addon_data (workaround for Kodi's settings
+      cache, useful for ad-hoc testing). Stable 360p path remains the
+      default until segment timing is solved.
 
 ## Upstream PR work (parallel lane — every bug evaluated for "fix it upstream?")