feat(host/windows): HDR scRGB→P010 in a shader — NVENC native P010, off the SM

On the Windows WGC HDR path the FP16 scRGB capture was fed to NVENC as
R10G10B10A2 (BT.2020 PQ), and NVENC did the RGB→YUV CSC internally on the
contended SM — adding to the encode_ms wall under a GPU-saturating game.
(NVIDIA's D3D11 VideoProcessor can't do RGB→P010 for HDR; that path renders
green, confirmed live — so the convert must be ours.)

New `HdrP010Converter` fuses the tone-map with the BT.2020 RGB→YUV matrix and
emits P010 (10-bit limited range) directly: a luma pass → an R16_UNORM plane
RTV (full-res) and a chroma pass → an R16G16_UNORM plane RTV (half-res, 2x2
box average) of a DXGI_FORMAT_P010 texture. NVENC then takes native P010 and
skips its SM-side convert.

Gated behind PUNKTFUNK_HDR_SHADER_P010 (default OFF → the existing
R10→NVENC path is byte-for-byte unchanged). Colour validated by a new
`hdr-p010-selftest` subcommand: a synthetic scRGB pattern → P010 → readback,
compared to a BT.2020 PQ 10-bit reference — max abs error Y=0.99 / Cb=0.82 /
Cr=0.75 codes on an RTX 4090. Live-validated HDR colours correct (no green).
Build + clippy (--features nvenc -D warnings) green on x86_64-pc-windows-msvc.

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

crates/punktfunk-host/src/main.rs

@@ -130,6 +130,13 @@ fn real_main() -> Result<()> {
         // `PUNKTFUNK_NV12` convert is colour-correct. Prints PASS/FAIL + max Y/U/V error.
         #[cfg(target_os = "linux")]
         Some("nv12-selftest") => zerocopy::nv12_selftest(),
+        // HDR P010 colour self-test (Windows; no display/capture needed): upload a known scRGB FP16
+        // pattern, run the `HdrP010Converter` shader → P010 on the GPU, read the Y/UV planes back, and
+        // compare against an f64 BT.2020-PQ limited-range reference. Validates the
+        // `PUNKTFUNK_HDR_SHADER_P010` colour math without green-screening a live HDR stream. Prints
+        // PASS/FAIL + max Y/Cb/Cr error.
+        #[cfg(target_os = "windows")]
+        Some("hdr-p010-selftest") => crate::capture::dxgi::hdr_p010_selftest(),
         // Compositor readiness probe: exit 0 iff the (detected or PUNKTFUNK_COMPOSITOR-forced)
         // compositor is up and able to create a virtual output *now*. A session-bringup
         // script polls this to gate on real readiness instead of a blind `sleep`.
@@ -551,6 +558,9 @@ NOTES:
         \x20   punktfunk-host service install    register an auto-start SYSTEM service + firewall rules\n\
         \x20   punktfunk-host service uninstall  remove the service + firewall rules\n\
         \x20   punktfunk-host service start|stop|status\n\
-        \x20   config: %ProgramData%\\punktfunk\\host.env"
+        \x20   config: %ProgramData%\\punktfunk\\host.env\n\
+        \nWINDOWS DIAGNOSTICS:\n\
+        \x20   punktfunk-host hdr-p010-selftest  GPU colour check for the PUNKTFUNK_HDR_SHADER_P010 path\n\
+        \x20                                     (scRGB FP16 -> P010 BT.2020 PQ shader vs an f64 reference)"
     );
 }