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/encode/sw.rs

@@ -151,6 +151,13 @@ impl Encoder for OpenH264Encoder {
             PixelFormat::Rgb10a2 => {
                 anyhow::bail!("software H.264 encoder cannot encode 10-bit HDR (Rgb10a2)")
             }
+            // NV12/P010 are GPU-resident video-processor outputs for the NVENC path; the software
+            // encoder never receives them (it only gets CPU RGB frames).
+            PixelFormat::Nv12 | PixelFormat::P010 => {
+                anyhow::bail!(
+                    "software encoder cannot encode YUV GPU textures (NV12/P010 → NVENC only)"
+                )
+            }
         }
 
         if self.force_kf {