feat(windows/capture): host-side cursor compositing for the capture model — the deterministic fix

Root cause, proven on-glass across five driver builds: a declared IddCx
hardware cursor is IRREVOCABLE. There is no un-declare DDI, the
empty-caps re-setup is rejected INVALID_PARAMETER, and after a
successful same-mode re-commit with the driver's re-declare provably
suppressed (sticky per-target flag, zero re-setups logged) DWM still
never composites the software cursor back into the monitor's frames.
Every DWM-based composite flip is therefore unfixable.

The composite (capture) model is now implemented in OUR pipeline:
- the driver keeps its hardware cursor declared for the session's whole
  life (the state that works) — frames stay pointer-free always;
- in composite mode try_consume routes the conversion through a blend
  scratch: slot copy + one alpha-blended quad of the GDI poller's shape
  at its polled position (CursorBlendPass — fullscreen-triangle VS with
  viewport placement, straight-alpha PS, sRGB→linear for FP16/HDR
  rings), covering all four convert paths (NV12, 4:4:4 copy, P010,
  PyroWave) GPU-side under the slot's keyed mutex;
- pointer-only motion produces no driver publish (declared hw cursor),
  so try_consume REGENERATES from the last slot whenever the polled
  cursor state changes — the cursor moves on a static desktop at tick
  rate, without faking freshness for the stall/death watchdogs;
- set_cursor_forward becomes purely host-internal state; the
  SET_CURSOR_FORWARD IOCTL machinery is no longer used by the host
  (driver keeps it, dormant) and its sender plumbing is removed.

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

crates/pf-capture/src/windows/dxgi.rs

@@ -161,7 +161,7 @@ pub fn install_gpu_pref_hook() {
     });
 }
 
-unsafe fn compile_shader(src: &str, entry: PCSTR, target: PCSTR) -> Result<Vec<u8>> {
+pub(crate) unsafe fn compile_shader(src: &str, entry: PCSTR, target: PCSTR) -> Result<Vec<u8>> {
     let mut blob: Option<ID3DBlob> = None;
     let mut errs: Option<ID3DBlob> = None;
     let r = D3DCompile(
@@ -194,7 +194,7 @@ unsafe fn compile_shader(src: &str, entry: PCSTR, target: PCSTR) -> Result<Vec<u
 }
 
 /// Fullscreen-triangle vertex shader for the HDR conversion pass (3 verts, no input layout).
-const HDR_VS: &str = r"
+pub(crate) const HDR_VS: &str = r"
 struct VOut { float4 pos : SV_POSITION; float2 uv : TEXCOORD0; };
 VOut main(uint vid : SV_VertexID) {
     float2 uv = float2((vid << 1) & 2, vid & 2);