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/libvpl-sys/src/lib.rs

@@ -0,0 +1,32 @@
+//! Raw FFI bindings to the vendored Intel VPL C API (`vpl/mfx.h`) plus the
+//! statically linked MIT dispatcher.
+//!
+//! Empty on targets other than Windows — see build.rs. The safe wrapper lives
+//! with its consumer (`pf-encode`'s `enc/windows/qsv.rs`); this crate is
+//! bindings only.
+
+#![allow(non_upper_case_globals)]
+#![allow(non_camel_case_types)]
+#![allow(non_snake_case)]
+// Bindgen output for a C API: u128 layout warnings and the like are upstream's concern.
+#![allow(improper_ctypes)]
+
+#[cfg(target_os = "windows")]
+include!(concat!(env!("OUT_DIR"), "/bindings.rs"));
+
+#[cfg(all(test, target_os = "windows"))]
+mod tests {
+    use super::*;
+
+    /// Link sanity: the static dispatcher resolves and its loader spins up
+    /// without an Intel driver present (enumeration may find zero
+    /// implementations — that's fine, MFXLoad itself must still succeed).
+    #[test]
+    fn dispatcher_links_and_loads() {
+        unsafe {
+            let loader = MFXLoad();
+            assert!(!loader.is_null(), "MFXLoad returned NULL");
+            MFXUnload(loader);
+        }
+    }
+}