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/pf-capture/src/linux/mod.rs

@@ -868,6 +868,10 @@ mod pipewire {
         /// Bumps whenever the bitmap (`rgba`/`bw`/`bh`) changes — stable across position-only moves,
         /// so the GPU encoder re-uploads its cursor texture only on change.
         serial: u64,
+        /// The compositor-reported hotspot — carried on the overlay for the cursor-forward
+        /// channel (the blend path uses the pre-adjusted `x`/`y` and never reads it).
+        hot_x: i32,
+        hot_y: i32,
     }
 
     impl CursorState {
@@ -884,6 +888,8 @@ mod pipewire {
                 h: self.bh,
                 rgba: self.rgba.clone(),
                 serial: self.serial,
+                hot_x: self.hot_x.max(0) as u32,
+                hot_y: self.hot_y.max(0) as u32,
             })
         }
     }
@@ -1380,6 +1386,8 @@ mod pipewire {
         cursor.visible = true;
         cursor.x = pos_x - hot_x;
         cursor.y = pos_y - hot_y;
+        cursor.hot_x = hot_x;
+        cursor.hot_y = hot_y;
         if bmp_off == 0 {
             // Position-only update — keep the cached bitmap.
             return;