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/plexPlayerHooks.subtitle.dom.test.tsx

@@ -16,7 +16,6 @@ import { createRef, type ReactNode, type RefObject } from "react";
 import { usePlexTracks } from "./plexPlayerHooks";
 import { api, type PlexItemDetail } from "./api";
 import { dismissError, getError } from "./errorDialog";
-import type Hls from "hls.js";
 
 const RK = "98614";
 
@@ -44,15 +43,13 @@ function mount(qc: QueryClient, { sessionK = 0, abs = 3600, loadStart = 0 } = {}
         sessionK,
         subOffset: 0,
         subPos: 88,
-        loadSession: async () => {},
+        restartSession: () => {},
+        setAudioTrack: () => false,
         patchPrefs: () => {},
         videoRef: createRef<HTMLVideoElement>(),
-        hlsRef: createRef<Hls>(),
-        multiAudioRef: { current: false },
         audioRef: { current: 0 } as RefObject<number | null>,
         aoffRef: { current: 0 },
-        absRef: { current: abs },
-        loadStartRef: { current: loadStart },
+        playhead: () => (abs > 0 ? abs : loadStart),
       }),
     { wrapper, initialProps: { id: RK } },
   );
@@ -320,15 +317,13 @@ function TrackHarness() {
     sessionK: 0,
     subOffset: 0,
     subPos: 88,
-    loadSession: async () => {},
+    restartSession: () => {},
+    setAudioTrack: () => false,
     patchPrefs: () => {},
     videoRef: createRef<HTMLVideoElement>(),
-    hlsRef: createRef<Hls>(),
-    multiAudioRef: { current: false },
     audioRef: { current: 0 } as RefObject<number | null>,
     aoffRef: { current: 0 },
-    absRef: { current: 3600 },
-    loadStartRef: { current: 0 },
+    playhead: () => 3600,
   });
   // eslint-disable-next-line react-hooks/immutability -- a TEST probe: the hook's return has to reach the assertions, and a render-phase ref write is the only way to read it from outside without a wrapper that would change what is being measured.
   harness.current = tracks;