feat(host+client): host-driven mouse-model flip — remote-desktop sweep M3

The full Parsec model: launching a game from a desktop session flips the
client into captured relative automatically, and back — no chords.

- Capture overlay now distinguishes 'hidden' from 'no cursor yet':
  CursorOverlay grows a visible flag, overlay() returns Some whenever a
  bitmap is known (the encode loop strips invisible overlays after
  forwarding so no blend path ever draws one; the CPU composite guarded
  on visibility already).
- Host forwarder maps it onto the reserved wire bit: visible ⇒ VISIBLE,
  hidden-but-known ⇒ RELATIVE_HINT (an app grabbed/hid the pointer),
  never any hint before the first bitmap — a cold start can't flip a
  desktop session into capture.
- Presenter: edge-triggered auto-flip on hint changes via the new
  Capture::set_desktop (chord semantics untouched); a manual ⌃⌥⇧M sets
  an override latch that holds until the HOST's intent next changes, so
  the hint never fights the user. Leaving relative warps the local
  cursor to the host's last pointer position through the new
  content_to_window inverse-letterbox mapping (unit-tested against
  finger_to_content) — the hand-back is seamless.

Verified on .21: fmt + clippy -D warnings + tests (presenter 20 incl.
the inverse-mapping roundtrip, host 245).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/pf-frame/src/lib.rs

@@ -201,6 +201,11 @@ pub struct CursorOverlay {
     /// client so a locally-drawn OS cursor points with the right pixel.
     pub hot_x: u32,
     pub hot_y: u32,
+    /// Compositor-reported pointer visibility. `false` = an app on the host grabbed/hid the
+    /// pointer — the cursor-forward channel turns that into the client's relative-mode hint
+    /// (remote-desktop-sweep M3). The encode loop STRIPS invisible overlays before the frame
+    /// reaches any blend path, so encoders may keep treating `Some` as "draw it".
+    pub visible: bool,
 }
 
 /// A captured frame. [`format`](Self::format)/dimensions describe the pixels regardless of