feat(plex): HW HDR→SDR tonemap for 10-bit/HDR transcode (OpenCL on the iGPU)

Upgrade mode-3's 10-bit/HDR branch from "route to software libx264" (BUG 1's
behaviour) to a full-GPU pipeline: VAAPI decode → OpenCL tonemap (hable,
HDR10/HLG → bt709 SDR nv12) → h264_vaapi. Gen 9.5 (UHD 630) can't tonemap in
the VAAPI VPP (tonemap_vaapi fails), so OpenCL is the only HW route — proven on
the prod iGPU at ~4.2x realtime, ~1 CPU core.

- stream.py: `_select_encode` returns (hw, tonemap); a 10-bit/HDR HW-decodable
  source picks the tonemap graph when the OpenCL runtime is present, else drops
  to software. `_opencl_present` gates on a registered ICD. `_venc_opts` /
  `_hwaccel_in_opts` gain the tonemap variant (named vaapi+opencl devices,
  `-filter_hw_device ocl`, hwmap→tonemap_opencl→hwmap reverse). `_want_hw` kept
  as the hw-half wrapper. The runtime HW→SW fallback still backstops a session
  that emits no segment.
- Dockerfile: add the Intel NEO OpenCL runtime (intel-opencl-icd). It was removed
  from Debian trixie, so it's pulled from bookworm (22.43 still supports Gen 9.5;
  the .deb runs on the trixie base — verified live). Harmless without /dev/dri.
- route + FE: thread `tonemap` through the session response; the stats overlay
  mode row reads "transcode · GPU · HDR→SDR" for the tonemap path.
- tests: tonemap command branch, the (hw, tonemap) selection matrix (auto/on ×
  8-bit/10-bit × opencl), start_session tonemap wiring, and the fmtMode tag.

frontend/src/lib/plexStats.ts

@@ -12,6 +12,9 @@ export interface PlexStatsInput {
   // software, null/undefined = not a transcode (or unknown) — so the mode row can read "transcode · GPU"
   // vs "transcode · CPU", the visible confirmation of which S1 path engaged.
   hwEncoder?: boolean | null;
+  // For a HW transcode of a 10-bit/HDR source: true = the GPU OpenCL tonemap path (HDR→SDR) engaged, so
+  // the mode row can read "transcode · GPU · HDR→SDR". Absent/false for an 8-bit HW or software session.
+  tonemap?: boolean | null;
   videoWidth: number; // intrinsic decoded dims (0 while the browser can't decode / pre-metadata)
   videoHeight: number;
   droppedFrames: number; // getVideoPlaybackQuality().droppedVideoFrames
@@ -94,20 +97,30 @@ export function fmtCodec(video: string | undefined, audio: string | undefined):
 }
 
 // Active mode, with the transcode encoder appended when known: "transcode · GPU" (VAAPI hardware) or
-// "transcode · CPU" (libx264 software) — the at-a-glance confirmation of which S1 path engaged. Non-
-// transcode modes (direct/remux) and an unknown encoder read as the bare mode. Raw tokens (not i18n'd),
-// consistent with the other technical values in this overlay (e.g. transport "hls").
-export function fmtMode(playable: string, hwEncoder?: boolean | null): string {
+// "transcode · CPU" (libx264 software) — the at-a-glance confirmation of which S1 path engaged — and a
+// "· HDR→SDR" tag when the GPU OpenCL tonemap path handled a 10-bit/HDR source ("transcode · GPU ·
+// HDR→SDR"). Non-transcode modes (direct/remux) and an unknown encoder read as the bare mode. Raw tokens
+// (not i18n'd), consistent with the other technical values in this overlay (e.g. transport "hls").
+export function fmtMode(
+  playable: string,
+  hwEncoder?: boolean | null,
+  tonemap?: boolean | null,
+): string {
   const mode = playable || DASH;
   if (playable !== "transcode" || hwEncoder == null) return mode;
-  return `${mode} · ${hwEncoder ? "GPU" : "CPU"}`;
+  const base = `${mode} · ${hwEncoder ? "GPU" : "CPU"}`;
+  return tonemap ? `${base} · HDR→SDR` : base;
 }
 
 // Build the ordered, i18n-keyed rows the overlay renders. Rows whose data is absent still show a dash
 // (honest "not available" beats a missing row) — except HLS-only rows, dropped entirely for direct play.
 export function buildStatRows(s: PlexStatsInput): StatRow[] {
   const rows: StatRow[] = [
-    { key: "mode", labelKey: "plex.player.stats.mode", value: fmtMode(s.playable, s.hwEncoder) },
+    {
+      key: "mode",
+      labelKey: "plex.player.stats.mode",
+      value: fmtMode(s.playable, s.hwEncoder, s.tonemap),
+    },
     { key: "transport", labelKey: "plex.player.stats.transport", value: s.transport },
     {
       key: "res",