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/components/PlexStatsOverlay.tsx

@@ -1,8 +1,8 @@
 import { type RefObject, useEffect, useState } from "react";
 import { useTranslation } from "react-i18next";
-import type Hls from "hls.js";
 import { X } from "lucide-react";
-import { buildStatRows, type FragMeter, type HlsLevelInfo, type StatRow } from "../lib/plexStats";
+import type { HlsEngine } from "../lib/hlsEngine";
+import { buildStatRows, type FragMeter, type StatRow } from "../lib/plexStats";
 import { IconButton } from "./ui/IconButton";
 
 // "Stats for nerds" debug overlay. Samples the live HTML5 <video> + hls.js state once a second
@@ -23,29 +23,24 @@ function frameCounts(v: HTMLVideoElement | null): { dropped: number; total: numb
 
 export function PlexStatsOverlay({
   videoRef,
-  hlsRef,
+  engine,
   fragMeterRef,
   audioLabel,
   subLabel,
   playable,
-  hwRef,
-  tonemapRef,
-  sessionStartRef,
   absRef,
   durationRef,
   onClose,
 }: {
   videoRef: RefObject<HTMLVideoElement | null>;
-  hlsRef: RefObject<Hls | null>;
+  // The session machine: the hls.js level, the transcode encoder and the session's start offset.
+  engine: Pick<HlsEngine, "levelInfo" | "hw" | "tonemap" | "sessionStart">;
   // Rolling FRAG_LOADED meter owned by the player (it lives as long as the hls.js instance, not as
   // long as this panel) — the source of the MEASURED bitrate row.
   fragMeterRef: RefObject<FragMeter>;
   audioLabel?: string; // selected audio track label, as the tracks menu names it
   subLabel?: string; // selected subtitle track label (or the menu's "Off" entry)
   playable: string;
-  hwRef: RefObject<boolean | null>; // active transcode encoder: true=GPU, false=CPU, null=not transcode
-  tonemapRef: RefObject<boolean | null>; // HW HDR→SDR tonemap engaged (adds a "· HDR→SDR" mode tag)
-  sessionStartRef: RefObject<number>;
   absRef: RefObject<number>;
   durationRef: RefObject<number>;
   onClose: () => void;
@@ -57,7 +52,6 @@ export function PlexStatsOverlay({
   useEffect(() => {
     const sample = () => {
       const v = videoRef.current;
-      const hls = hlsRef.current;
 
       const { dropped, total } = frameCounts(v);
 
@@ -72,29 +66,15 @@ export function PlexStatsOverlay({
         }
       }
 
-      let level: HlsLevelInfo | null = null;
-      if (hls) {
-        // currentLevel is -1 while auto-ABR hasn't settled; fall back to loadLevel for the res/bitrate.
-        const idx = hls.currentLevel >= 0 ? hls.currentLevel : hls.loadLevel;
-        const lv = idx >= 0 ? hls.levels?.[idx] : undefined;
-        level = {
-          index: hls.currentLevel,
-          total: hls.levels?.length ?? 0,
-          width: lv?.width,
-          height: lv?.height,
-          bitrate: lv?.bitrate,
-          videoCodec: lv?.videoCodec,
-          audioCodec: lv?.audioCodec,
-        };
-      }
+      const level = engine.levelInfo();
 
       setRows(
         buildStatRows({
           lang,
           playable,
-          hwEncoder: hwRef.current,
-          tonemap: tonemapRef.current,
-          transport: hls ? "hls" : "direct",
+          hwEncoder: engine.hw,
+          tonemap: engine.tonemap,
+          transport: level ? "hls" : "direct",
           videoWidth: v?.videoWidth ?? 0,
           videoHeight: v?.videoHeight ?? 0,
           droppedFrames: dropped,
@@ -104,7 +84,7 @@ export function PlexStatsOverlay({
           measuredBps: fragMeterRef.current?.bitrate(),
           audioLabel,
           subLabel,
-          sessionStartS: sessionStartRef.current ?? 0,
+          sessionStartS: engine.sessionStart.current,
           abs: absRef.current ?? 0,
           duration: durationRef.current ?? 0,
         }),
@@ -113,20 +93,7 @@ export function PlexStatsOverlay({
     sample();
     const iv = setInterval(sample, 1000);
     return () => clearInterval(iv);
-  }, [
-    lang,
-    videoRef,
-    hlsRef,
-    fragMeterRef,
-    audioLabel,
-    subLabel,
-    playable,
-    hwRef,
-    tonemapRef,
-    sessionStartRef,
-    absRef,
-    durationRef,
-  ]);
+  }, [lang, videoRef, engine, fragMeterRef, audioLabel, subLabel, playable, absRef, durationRef]);
 
   return (
     <div