feat(encode): native QSV backend — libvpl-sys + qsv.rs (Phases 0-3 of design/native-qsv-encoder.md)

Vendored MIT VPL dispatcher (static, trimmed tree, pin 674d015b/v2.17.0) built
via cmake+bindgen behind new feature 'qsv' (pf-encode + punktfunk-host forward).
qsv.rs: dispatcher session on the capture adapter (LUID-matched), SetHandle
D3D11, AsyncDepth=1/GopRefDist=1/VDEnc/CBR + HRD-off low-latency config,
GetSurfaceForEncode + GPU CopySubresourceRegion input (zero-copy, no readback
path), bounded sync-point poll, in-place reset with teardown escalation, no-IDR
bitrate retarget (Reset + StartNewSequence=OFF), 10-bit P010 HEVC-Main10/AV1,
HDR mastering/CLL SEI-OBU at IDR + BT.2020/PQ VSI, LTR-RFI via mfxExtRefListCtrl
(AMF slot policy port, Query-gated per codec, wire-index FrameOrder pinning).
Dispatch: native-first with ffmpeg fallback + PUNKTFUNK_QSV_FFMPEG hatch;
probes (can_encode_10bit / windows_codec_support / windows_backend_is_probed)
now answer natively for QSV.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/pf-encode/Cargo.toml

@@ -51,6 +51,8 @@ nvidia-video-codec-sdk = { version = "0.4", features = ["ci-check"], optional =
 ffmpeg-next = { version = "8", optional = true }
 # `libnvidia-encode`/`nvEncodeAPI64.dll` resolved at runtime; the NVENC status→cause table dlopen.
 libloading = "0.8"
+# Native Intel QSV (VPL): vendored static MIT dispatcher + bindgen'd C API, only under `qsv`.
+libvpl-sys = { path = "../libvpl-sys", optional = true }
 windows = { version = "0.62", features = [
     "Win32_Foundation",
     "Win32_Graphics_Direct3D",
@@ -72,3 +74,8 @@ amf-qsv = ["dep:ffmpeg-next"]
 vulkan-encode = []
 # PyroWave — the opt-in wired-LAN intra-only wavelet codec (Linux encode backend).
 pyrowave = ["dep:pyrowave-sys"]
+# Native Intel QSV via the statically linked VPL dispatcher (Windows; runtime GPU
+# libs come from the Intel driver store). Supersedes the ffmpeg `amf-qsv` QSV path
+# (design/native-qsv-encoder.md). ⚠ Like `nvenc`: hand builds need this feature or
+# Intel boxes fall through to the ffmpeg path / software.
+qsv = ["dep:libvpl-sys"]