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/handshake.rs

@@ -8,6 +8,22 @@
 
 use super::*;
 
+/// Whether this session forwards the cursor out-of-band (design/remote-desktop-sweep.md M2):
+/// the client asked ([`CLIENT_CAP_CURSOR`](punktfunk_core::quic::CLIENT_CAP_CURSOR)) AND the
+/// capture path can deliver cursor metadata separately from the frame — today that is the
+/// Linux portal `SPA_META_Cursor` path only: not gamescope (its capture paints no cursor at
+/// all), not Windows (DWM composites into the IDD frame — M2c). THE single predicate: the
+/// Welcome's `HOST_CAP_CURSOR` bit and the session's forwarding/blend-off wiring both read it,
+/// so they can never disagree.
+pub(super) fn cursor_forward(
+    client_caps: u8,
+    compositor: Option<crate::vdisplay::Compositor>,
+) -> bool {
+    cfg!(target_os = "linux")
+        && client_caps & punktfunk_core::quic::CLIENT_CAP_CURSOR != 0
+        && compositor.is_some_and(|c| c != crate::vdisplay::Compositor::Gamescope)
+}
+
 /// Run the Hello→Welcome→Start negotiation. Borrows the control streams (the caller keeps them for
 /// mid-stream renegotiation afterwards). `first` is the already-read first control message.
 #[allow(clippy::type_complexity, clippy::too_many_arguments)]
@@ -453,6 +469,14 @@ pub(super) async fn negotiate(
                 punktfunk_core::quic::HOST_CAP_TEXT_INPUT
             } else {
                 0
+            }
+            // Cursor channel granted (client asked + this capture path can deliver cursor
+            // metadata out of the frame) — the client turns its local renderer on ONLY when
+            // it sees this bit, and serve_session wires forwarding from the same predicate.
+            | if cursor_forward(hello.client_caps, compositor) {
+                punktfunk_core::quic::HOST_CAP_CURSOR
+            } else {
+                0
             },
         // The negotiated session AEAD (resolved above) + its 32-byte key toward a ChaCha
         // client; toward everyone else cipher 0 keeps the Welcome byte-identical to the