feat(codec): use SVT-AV1 for AV1 encoding on 64-bit targets

Encode AV1 with SVT-AV1 v4.2.0 (rtc + low delay + CBR) instead of libaom
on all 64-bit targets; 32-bit targets keep aom. Decoding is unchanged and
the wire format is still AV1, so old peers are unaffected.

- vcpkg overlay port pinned to v4.2.0: static build, LTO off (mixed
  toolchain safety), MSVC >= 1950 inline workaround, no forced llvm
  binutils
- low delay contract: one packet out per picture in, blocking get_packet
- bitrate and frame rate change on the fly via RATE_CHANGE_EVENT /
  FRAME_RATE_CHANGE_EVENT; VideoQoS pushes its pacing rate through the
  new EncoderApi::set_fps, applied only after a successful send
- fall back to aom for i444 (svt-av1 is 4:2:0 only), for unsupported
  resolutions and on encoder init failure
- test_av1 now gates auto AV1 on the encoder actually used, the cached
  verdict is re-keyed (SVT-Y/SVT-N) so upgrades re-measure
- unit tests: per-frame packet contract, aom cross-decode of the
  bitstream, on-the-fly event acceptance

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012Wf7v1KJM6mbhiYxWSz4PH

src/server/video_service.rs

@@ -1023,12 +1023,28 @@ fn get_encoder_config(
             },
             keyframe_interval,
         }),
-        CodecFormat::AV1 => EncoderCfg::AOM(AomEncoderConfig {
-            width: c.width as _,
-            height: c.height as _,
-            quality,
-            keyframe_interval,
-        }),
+        CodecFormat::AV1 => {
+            let aom = EncoderCfg::AOM(AomEncoderConfig {
+                width: c.width as _,
+                height: c.height as _,
+                quality,
+                keyframe_interval,
+            });
+            // Prefer svt-av1 for AV1 encoding, keep aom for i444 (svt-av1 is 4:2:0
+            // only) and for resolutions svt-av1 cannot handle.
+            #[cfg(target_pointer_width = "64")]
+            if scrap::svt_av1::SvtAv1Encoder::support(c.width as _, c.height as _)
+                && !Encoder::use_i444(&aom)
+            {
+                return EncoderCfg::SVTAV1(scrap::svt_av1::SvtAv1EncoderConfig {
+                    width: c.width as _,
+                    height: c.height as _,
+                    quality,
+                    keyframe_interval,
+                });
+            }
+            aom
+        }
         _ => EncoderCfg::VPX(VpxEncoderConfig {
             width: c.width as _,
             height: c.height as _,
@@ -1326,6 +1342,9 @@ fn check_qos(
 ) -> ResultType<()> {
     let mut video_qos = VIDEO_QOS.lock().unwrap();
     *spf = video_qos.spf();
+    // encoders whose rate control budgets per frame need the real pacing rate,
+    // implementations no-op when the value is unchanged
+    encoder.set_fps(video_qos.fps());
     if *ratio != video_qos.ratio() {
         *ratio = video_qos.ratio();
         if encoder.support_changing_quality() {