feat(windows-host): IDD-push encodes native NV12/P010 (skip NVENC's SM-side CSC)

GPU-contention work (host-latency plan §5.A): the IDD-push output ring now hands
NVENC native YUV instead of RGB, so NVENC skips its internal RGB→YUV colour
conversion on the SM/3D engine the running game saturates.

- idd_push.rs: out_ring is now NV12 (SDR, BT.709 limited) via a D3D11 VIDEO-engine
  BGRA→NV12 VideoConverter (keeps the CSC off the contended 3D/compute engine), or
  P010 (HDR, BT.2020 PQ limited) via the FP16→P010 shader (NVIDIA's VideoProcessor
  can't do RGB→P010). The ring drops its per-slot RTV (textures only), matching the
  WGC YUV ring; converters rebuild on a size/HDR flip.
- nvenc.rs: NV12 input forces bit_depth=8 so an HDR→SDR toggle (or a 10-bit-
  negotiated client on an SDR display) re-inits the session at the matching depth —
  NV12 can't feed a 10-bit session (register_resource rejects it).
- punktfunk1.rs: per-stage latency instrumentation under PUNKTFUNK_PERF
  (cap=try_latest, submit=encode_picture, wait=lock_bitstream µs p50/p99/max) to
  pinpoint where capture→encoded latency goes under GPU saturation.

crates/punktfunk-host/src/encode/windows/nvenc.rs

@@ -609,7 +609,14 @@ impl Encoder for NvencD3d11Encoder {
                     self.bit_depth = 10;
                     nv::NV_ENC_BUFFER_FORMAT::NV_ENC_BUFFER_FORMAT_ABGR10
                 }
-                PixelFormat::Nv12 => nv::NV_ENC_BUFFER_FORMAT::NV_ENC_BUFFER_FORMAT_NV12,
+                PixelFormat::Nv12 => {
+                    // NV12 is 8-bit 4:2:0. Force 8-bit so a transition from a prior P010 (10-bit) session
+                    // — or a 10-bit-negotiated client on an SDR display — re-inits at the matching depth.
+                    // Unlike ARGB (which NVENC upconverts to Main10), NV12 cannot feed a 10-bit session:
+                    // `register_resource` rejects it as InvalidParam (the HDR→SDR-toggle stream drop).
+                    self.bit_depth = 8;
+                    nv::NV_ENC_BUFFER_FORMAT::NV_ENC_BUFFER_FORMAT_NV12
+                }
                 _ => nv::NV_ENC_BUFFER_FORMAT::NV_ENC_BUFFER_FORMAT_ARGB,
             };
             let device = frame.device.clone();