feat(host): native AMF SDK encoder for Windows AMD — drop libavcodec

Direct-SDK AMF encoder (encode/windows/amf.rs), the AMD analogue of the
direct-NVENC path, replacing the libavcodec *_amf dispatch. C-vtable FFI
pinned to AMF headers v1.4.36, runtime-loaded from the driver's amfrt64.dll
(no build feature, no new dependency) exactly as NVENC loads its DLL.

- AVC/HEVC (SDR NV12 + 10-bit HDR P010) and AV1 (RDNA3+, probed); a bounded
  poll retires the libavcodec ~2-frame output hold; native in-place reset().
- Intra-refresh wave (PUNKTFUNK_INTRA_REFRESH), in-band HDR mastering/CLL
  metadata (*InHDRMetadata -> HEVC SEI / AV1 OBU), and a native codec probe
  feeding the GameStream advertisement (windows_backend_is_ffmpeg ->
  windows_backend_is_probed).
- AMD dispatch / advertisement / 4:4:4 are native-only; the libavcodec AMF
  fallback and the PUNKTFUNK_AMF_FFMPEG hatch are removed. FFmpeg serves QSV
  only (its AMF path retained solely as the latency A/B comparator).
- Overload back-pressure: submit bounds in-flight surfaces below the input
  ring, draining finished AUs (buffered for poll, FIFO-preserved) to free a
  slot and retry on AMF_INPUT_FULL instead of tearing the encoder down and
  forcing an IDR; this also closes a latent ring-overwrite corruption seen
  under load on-glass.

Validated on the lab Ryzen iGPU (AMF runtime 1.4.37): HEVC/AVC across a
native reset, HEVC Main10 mastering+CLL SEIs byte-verified, intra-refresh
accepted, a backpressure burst FIFO-clean, and end-to-end via the macOS
client. Measured §5.2 latency A/B: native encode_us p50 ~5 ms (0.31 frame
periods) vs libavcodec ~17 ms (1.01). 4:4:4 stays unsupported (VCN hardware
limit). Live-gated tests skip cleanly on non-AMD boxes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/gamestream/serverinfo.rs

@@ -77,9 +77,10 @@ fn base_codec_mode_support() -> u32 {
         }
     }
     // Windows AMD/Intel (AMF/QSV): advertise only what the GPU actually encodes (AV1 is narrow, an
-    // old iGPU might lack HEVC). NVENC and the GPU-less software path keep the static superset.
-    #[cfg(all(target_os = "windows", feature = "amf-qsv"))]
-    if crate::encode::windows_backend_is_ffmpeg() {
+    // old iGPU might lack HEVC). AMF probes natively (no build feature needed); QSV needs the
+    // libavcodec build. NVENC and the GPU-less software path keep the static superset.
+    #[cfg(target_os = "windows")]
+    if crate::encode::windows_backend_is_probed() {
         if let Some(m) = probed_mask(crate::encode::windows_codec_support()) {
             return m;
         }
@@ -91,7 +92,7 @@ fn base_codec_mode_support() -> u32 {
 /// or `None` if the probe found nothing — meaning the GPU wasn't usable at probe time (GPU-less CI,
 /// a misconfigured/wrong-vendor host), NOT that it encodes zero codecs; the caller then advertises
 /// the static superset (pre-probe behaviour) rather than claiming nothing.
-#[cfg(any(target_os = "linux", all(target_os = "windows", feature = "amf-qsv")))]
+#[cfg(any(target_os = "linux", target_os = "windows"))]
 fn probed_mask(caps: crate::encode::CodecSupport) -> Option<u32> {
     use super::{SCM_AV1_MAIN8, SCM_H264, SCM_HEVC};
     let mut m = 0;