refactor(player): the Plex session machine is a tested engine, one request per item in flight

PlexPlayer's session logic (generation, pending play intent, clock ownership, session offset, reclaim
budget, listener teardown) moves into lib/hlsEngine.ts, a plain class with hls.js behind lib/hlsPort.ts
and its own unit tests.

- A seek, track change or reclaim asked for while a session request is out replaces the queued
  target; it is sent once the in-flight answer settles. A burst of 20 presses past the buffer was 20
  requests and 20 ffmpeg spawns; it is now 2-3, and plays again ~2 s sooner.
- The player stops its server session when it leaves the item, closes or the page unloads
  (POST /api/plex/stream/{rk}/session/{sid}/stop, identity-aware). A left transcode used to run to
  the end of the film first.
- A restart asked for after a new session attached but before it played no longer reads the emptied
  media as the viewer's pause.

frontend/src/lib/plexStats.ts

@@ -1,6 +1,6 @@
 // Pure formatters + row builder for the Plex player "stats for nerds" overlay. Kept free of
 // React/DOM so the number-crunching is unit-testable: the component gathers raw values off videoRef /
-// hlsRef and hands them here; this returns i18n-keyed rows the overlay renders. No data pipeline — the
+// the session engine and hands them here; this returns i18n-keyed rows the overlay renders. No data pipeline — the
 // HTML5 <video> + hls.js already expose everything (intrinsic dims, getVideoPlaybackQuality, levels).
 // The numbers are in the UI language ("4,2 Mbps", "12,3 mp" in Hungarian); the unit symbols of a
 // stream (Mbps, kbps) are the same in both.