DASH HD playback — WIP behind TORTTUBE_DASH=1 + upstream notes

Sidecar resolve_dash op shipped — returns rustypipe's full video_only_streams
+ audio_streams (16+ representations for NGGYU, from 360p H.264 through 4K
AV1). Addon _build_dash_mpd assembles a valid on-demand MPEG-DASH manifest
filtered to H.264 ≤1080p + best AAC audio.

Two unblocked-by-WIP issues surfaced during integration:
- inputstream.adaptive's libcurl can't open file:// URLs (logged in
  docs/upstream.md as an enhancement-target).
- Rapid Player.Open retries can trigger Kodi's 'two concurrent
  busydialogs' fatal exit; need lifecycle hardening before re-enabling.

Pivoted to localhost HTTP-server serving (ThreadingHTTPServer on a
port-0 socket, MPD bytes captured in a per-instance handler subclass).
Lifecycle: server.shutdown() runs in a finally block after the
SponsorBlockMonitor watcher exits. Works in isolation but Kodi crashed
under rapid retry conditions — needs more testing.

For v0.0.5: DASH path is gated behind TORTTUBE_DASH=1 env var; default
falls through to the stable yt-dlp progressive 360p path that's been
verified live. M7 milestone added to track the remaining work; PRs
to inputstream.adaptive + Kodi candidates logged in docs/upstream.md.

MILESTONES.md

@@ -71,8 +71,25 @@
       picked format 18, 360p H.264+AAC progressive)
 - [ ] armv7 build for older Pis (deferred — current TV is aarch64-capable
       and my static sidecar runs on it even though userspace is armhf)
-- [ ] 720p+ playback (deferred to M5+) — itag 22 (720p progressive) is
-      deprecated by YouTube; higher quality needs DASH manifest generation
+- [ ] 720p+ playback (DASH WIP — see "M7 — DASH/HD" below) — itag 22 (720p
+      progressive) is deprecated by YouTube; higher quality needs DASH
+      manifest generation. Code path exists, gated behind `TORTTUBE_DASH=1`.
+
+## M7 — DASH / HD playback [WIP]
+
+- [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
 
 ## Upstream PR work (parallel lane — every bug evaluated for "fix it upstream?")