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>

clients/android/native/src/session/connect.rs

@@ -201,6 +201,9 @@ pub extern "system" fn Java_io_unom_punktfunk_kit_NativeBridge_nativeConnect<'lo
         // No display-volume forwarding from Android yet (the panel tone-maps PQ itself via the
         // Surface dataspace + static metadata) — the host keeps its virtual-display EDID defaults.
         None,
+        // No non-video caps: this client does not render the host cursor locally (no shape/state
+        // planes in the jni surface), so advertising CLIENT_CAP_CURSOR would stream cursor-less.
+        0,
         launch,   // a store-qualified library id to boot into a game, or None for the desktop
         pin,      // Some → Crypto on host-fp mismatch
         identity, // owned (cert, key) PEM, or None (anonymous)

clients/linux/src/app.rs

@@ -547,6 +547,7 @@ impl AppModel {
                     crate::video::decodable_codecs(), // codecs (unused by the probe, but honest)
                     0,                                // preferred_codec: no preference
                     None, // display_hdr: probe connect, nothing presents
+                    0,    // client_caps: probe connect, nothing renders a cursor
                     None, // launch: probe connect, no game
                     pin,
                     Some(identity),

clients/probe/src/main.rs

@@ -523,6 +523,9 @@ async fn session(args: Args) -> Result<()> {
             // writes it into the virtual display's EDID (CTA HDR block), so the EDID-forwarding
             // path can be validated headlessly (check the host's monitor caps / ADD log line).
             display_hdr: punktfunk_core::client::display_hdr_env_override(),
+            // No CLIENT_CAP_CURSOR: this headless tool renders nothing — advertising it would
+            // just strip the pointer from the dumped bitstream.
+            client_caps: 0,
         }
         .encode(),
     )

clients/session/src/main.rs

@@ -172,6 +172,11 @@ mod session_main {
             // defaults for Linux clients; `PUNKTFUNK_CLIENT_PEAK_NITS` (read in the session
             // pump) pins one manually.
             display_hdr: None,
+            // The presenter renders the host cursor locally in desktop mouse mode (M2 cursor
+            // channel); capture-mode sessions keep the composited cursor, so only advertise
+            // when the session STARTS in desktop mode. The host gates further (Linux portal
+            // compositors only).
+            cursor_forward: settings.mouse_mode() == trust::MouseMode::Desktop,
             mic_enabled: settings.mic_enabled,
             clipboard,
             // The Settings preference (auto → VAAPI where it exists; the presenter

clients/windows/src/probe.rs

@@ -56,6 +56,7 @@ pub fn run_speed_probe(
         decodable_codecs(),
         0,    // preferred_codec: no preference
         None, // display_hdr: probe connect, nothing presents
+        0,    // client_caps: probe connect, nothing renders a cursor
         None, // launch: no game
         pin,
         Some(identity),