feat(core+host+driver+client): mid-stream cursor-render flip — host composites in the capture model

The cursor channel excluded the host pointer from the video for the
whole session, but the client only draws it under the DESKTOP mouse
model — flipping ⌃⌥⇧M to capture mid-stream left the session
pointer-blind (user-found). The fix makes render ownership LIVE state:

- wire: CursorRenderMode 0x51 (client→host, control stream) —
  client_draws: true = desktop model (host excludes + forwards),
  false = capture model / released (host composites, full fidelity —
  DWM on Windows incl. real XOR inversion, encoder blend on Linux).
  Sessions start client_draws=true (the pre-message behavior).
- host: control task stores the flag; the encode loop edge-detects it —
  forwarding + frame.cursor strip while the client draws, quiet
  forwarder + overlay-into-blend while composited. SessionPlan now
  grants blend CAPABILITY wherever the capture has a pointer (dropping
  the !cursor_forward gate) so the composite side needs no encoder
  rebuild; per-tick frame.cursor decides what is drawn.
- Windows: Capturer::set_cursor_forward → retained
  IOCTL_SET_CURSOR_FORWARD sender (proto v6) → driver (un)declares the
  IddCx hardware cursor on the LIVE monitor. Un-declare re-issues the
  setup with EMPTY caps (candidate mechanism — the DDI has no
  documented un-setup); resetup-on-commit is skipped while off, so a
  mode commit's software-cursor default is the fallback path. Failure
  against a pre-v6 driver logs and keeps declared-at-ADD behavior.
- SDL client: one edge-detected reconciler at the cursor pump site —
  desktop-active = client draws; capture model or released = host
  composites. Covers the chord, the M3 auto-flip, and engage/release.
- C ABI v12: punktfunk_connection_set_cursor_render (additive; wire
  version unchanged — pre-§8 hosts ignore the unknown message type).

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

crates/pf-vdisplay/src/vdisplay/windows/pf_vdisplay.rs

@@ -258,6 +258,31 @@ pub unsafe fn send_cursor_channel(
     .context("pf-vdisplay SET_CURSOR_CHANNEL")
 }
 
+/// Flip a LIVE monitor's hardware-cursor declaration (`IOCTL_SET_CURSOR_FORWARD`, proto v6) —
+/// the mid-stream cursor-render flip. Fails against a pre-v6 driver (unknown IOCTL); callers
+/// log and keep the declared-at-ADD behavior.
+///
+/// # Safety
+/// `dev` must be a live pf-vdisplay control handle (see [`super::manager::control_device_handle`]).
+pub unsafe fn send_cursor_forward(
+    dev: HANDLE,
+    req: &control::SetCursorForwardRequest,
+) -> Result<()> {
+    let mut none: [u8; 0] = [];
+    // SAFETY: per this fn's contract `dev` is the live control handle; `bytes_of(req)` borrows the
+    // caller's request across this synchronous call; no output buffer.
+    unsafe {
+        ioctl(
+            dev,
+            control::IOCTL_SET_CURSOR_FORWARD,
+            bytemuck::bytes_of(req),
+            &mut none,
+        )
+    }
+    .map(|_| ())
+    .context("pf-vdisplay SET_CURSOR_FORWARD")
+}
+
 /// RAII over a SetupAPI device-info list: every exit path of [`open_device`] destroys it (the error
 /// paths used to leak one `HDEVINFO` per failed open — and a driverless / mid-upgrade box probes
 /// repeatedly).