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.test.ts

@@ -118,6 +118,11 @@ describe("buildStatRows", () => {
     expect(cpu.find((r) => r.key === "mode")?.value).toBe("transcode · CPU");
   });
 
+  it("tags the mode row with HDR→SDR for a HW tonemap session", () => {
+    const rows = buildStatRows({ ...base, playable: "transcode", hwEncoder: true, tonemap: true });
+    expect(rows.find((r) => r.key === "mode")?.value).toBe("transcode · GPU · HDR→SDR");
+  });
+
   it("shows an empty classification honestly as a dash", () => {
     expect(buildStatRows({ ...base, playable: "" }).find((r) => r.key === "mode")?.value).toBe(
       DASH,
@@ -133,5 +138,9 @@ describe("fmtMode", () => {
     expect(fmtMode("transcode")).toBe("transcode");
     expect(fmtMode("remux", true)).toBe("remux"); // not a transcode → no encoder suffix
     expect(fmtMode("")).toBe(DASH);
+    // The HDR→SDR tag rides on the GPU encoder token for a tonemap session.
+    expect(fmtMode("transcode", true, true)).toBe("transcode · GPU · HDR→SDR");
+    expect(fmtMode("transcode", true, false)).toBe("transcode · GPU");
+    expect(fmtMode("transcode", null, true)).toBe("transcode"); // no encoder → no tag either
   });
 });