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/punktfunk-host/src/capture.rs

@@ -174,26 +174,6 @@ pub fn capture_virtual_output(
             },
         ) as pf_capture::CursorChannelSender
     });
-    // The mid-stream cursor-render flip (proto v6) — retained by the capturer so the client's
-    // mouse-model chords can (un)declare the driver's hardware cursor live. Same facade contract
-    // as `cursor_sender`.
-    let cursor_forward_sender: Option<pf_capture::CursorForwardSender> =
-        want.hw_cursor.then(|| {
-            std::sync::Arc::new(
-                move |req: &pf_driver_proto::control::SetCursorForwardRequest| {
-                    // SAFETY: `control_raw` is the pf-vdisplay control handle resolved above; it is
-                    // never closed for the process lifetime (`send_cursor_forward`'s precondition).
-                    unsafe {
-                        crate::vdisplay::driver::send_cursor_forward(
-                            windows::Win32::Foundation::HANDLE(
-                                control_raw as *mut core::ffi::c_void,
-                            ),
-                            req,
-                        )
-                    }
-                },
-            ) as pf_capture::CursorForwardSender
-        });
     pf_capture::open_idd_push(
         target,
         pref,
@@ -203,7 +183,6 @@ pub fn capture_virtual_output(
         keep,
         sender,
         cursor_sender,
-        cursor_forward_sender,
     )
     .map_err(|(e, _keep)| e.context("IDD-push capture open (no fallback)"))
 }