feat(core+host+client): cursor channel — remote-desktop sweep M2a+M2b

The host cursor stops riding the video and becomes a real OS cursor on
the client (the Parsec/RDP model): pointer feel no longer pays the
capture→encode→network→decode→present round trip.

Wire (M2a):
- Hello grows a client_caps trailing byte (CLIENT_CAP_CURSOR) after the
  fixed display_hdr block — presence disambiguated by remaining length,
  which caps the post-HDR tail at 27 bytes (documented); Welcome answers
  HOST_CAP_CURSOR (capable-and-asked, the 444/clipboard precedent).
- CursorShape (0x50, control stream): serial + dims + hotspot + straight
  RGBA, ≤120px/side so the u16 frame always fits (128² would overshoot);
  client caches by serial — re-showing a known shape costs 14 bytes, not
  a bitmap (RDP pointer-cache for free).
- CursorState (0xD0 datagram): serial + visible/relative_hint flags +
  position, sent once per encode-loop tick — latest-wins, self-healing
  under loss, no refresh timer. relative_hint is reserved for M3.
- Client core: two new planes (control-task + datagram-task arms) →
  next_cursor_shape/next_cursor_state; connect() grows client_caps
  (C ABI passes 0 until the v11 cursor poll fns exist).

Host (M2b, Linux portal only):
- handshake::cursor_forward is THE predicate (client asked ∧ Linux ∧
  compositor ≠ gamescope) — Welcome bit and session wiring both read it.
- SessionPlan.cursor_blend goes false for a forwarding session; the
  encode loop ticks a CursorForwarder every iteration: shape-serial diff
  → control-task bridge (mirrors probe_result), state datagram → conn.
- CursorOverlay/capture CursorState carry the hotspot through
  (nearest-neighbor downscale backstop for XL cursors, unit-tested).

Presenter:
- CursorChannel drains both planes per loop iteration; shapes become
  SDL color cursors (from_surface + hotspot), applied while the desktop
  mouse model is engaged; visibility follows the host; capture/released
  hands back the system cursor. Sessions advertise the cap when they
  START in desktop mode.

Verified on .21: fmt + clippy -D warnings (7 crates) + tests green
(core 218 incl. new wire roundtrips, host 245 incl. e2e + forwarder
downscale tests).

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

crates/punktfunk-host/src/native/control.rs

@@ -30,6 +30,7 @@ pub(super) async fn run(
     probe_tx: std::sync::mpsc::Sender<ProbeRequest>,
     mut probe_result_rx: tokio::sync::mpsc::UnboundedReceiver<ProbeResult>,
     mut reconfig_result_rx: tokio::sync::mpsc::UnboundedReceiver<Reconfigured>,
+    mut cursor_shape_rx: tokio::sync::mpsc::UnboundedReceiver<punktfunk_core::quic::CursorShape>,
     clip_enabled: Arc<AtomicBool>,
     clip: pf_clipboard::ClipCoord,
 ) {
@@ -262,6 +263,15 @@ pub(super) async fn run(
                     break;
                 }
             }
+            shape = cursor_shape_rx.recv() => {
+                // Cursor-forward bridge (M2): the encode loop diffed a new pointer bitmap.
+                // Rare (shape changes are human-paced); ≤ ~58 KiB fits the u16 frame by
+                // construction (cursor_fwd downscales).
+                let Some(shape) = shape else { break }; // data plane gone
+                if io::write_msg(&mut ctrl_send, &shape.encode()).await.is_err() {
+                    break;
+                }
+            }
             offer = clip_offer_rx.recv(), if !clip_offer_closed => {
                 // Host copied → the coordinator minted a `ClipOffer`; forward it to the client
                 // (only while sync is on — a race with a just-received disable would otherwise