perf(host/windows): move capture→encode off the 3D engine (NV12/P010 video-processor path, zero-copy, GPU priority)

The Windows host capped at ~60 fps with 35-40 ms latency on a GPU-heavy game:
the per-frame capture→encode path shared the 3D engine with the game and got
scheduled behind it. Rework to minimize 3D-engine work per frame:

- VideoConverter (D3D11 video processor): capture → NVENC-native NV12/P010 so
  NVENC skips its internal RGB→YUV (a 3D/compute step). Wired into both DDA
  (dxgi.rs) and WGC (wgc.rs). New PixelFormat::Nv12/P010 + NVENC YUV input.
- GPU scheduling hardening (Apollo-style): D3DKMTSetProcessSchedulingPriorityClass
  HIGH, absolute SetGPUThreadPriority, SetMaximumFrameLatency(1).
- WGC SDR zero-copy (hold pool frames; no CopyResource). DDA keeps a fast
  CopyResource to decouple its single-frame acquire/release from the async convert.
- Pipelined helper encode loop (PUNKTFUNK_ENCODE_DEPTH, default 1) + perf split
  (cap_wait / encode / write).

Live on the RTX 4090: hard 60 fps ceiling removed (now scene-scaling 40-200+),
latency much reduced. Residual cap in GPU-pinned scenes is the irreducible RGB→YUV
convert (no fixed-function unit on NVIDIA — VideoProcessing engine reads 0%) waiting
behind an uncapped game under WDDM context time-slicing; Linux avoids it via
gamescope capping the game to the display refresh.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/punktfunk-host/src/capture.rs

@@ -26,6 +26,13 @@ pub enum PixelFormat {
     /// produces this: scRGB FP16 desktop pixels are converted to BT.2020 PQ and written here, then
     /// handed to NVENC as `ABGR10` for an HEVC Main10 / HDR10 encode.
     Rgb10a2,
+    /// `NV12` (DXGI `NV12`): 8-bit BT.709 limited-range YUV 4:2:0. Produced by the D3D11 **video
+    /// processor** (video engine, not the 3D engine) so the per-frame colour conversion doesn't fight a
+    /// GPU-saturating game; handed to NVENC as `NV12` (it encodes YUV natively — no internal RGB→YUV).
+    Nv12,
+    /// `P010` (DXGI `P010`): 10-bit BT.2020 PQ limited-range YUV 4:2:0. HDR analogue of [`Nv12`]:
+    /// video-processor output for HEVC Main10 / HDR10, handed to NVENC as `YUV420_10BIT`.
+    P010,
 }
 
 impl PixelFormat {