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-core/src/quic/caps.rs

@@ -71,6 +71,24 @@ pub const HOST_CAP_GAMEPAD_STATE: u8 = 0x01;
 /// trailing `host_caps` byte — no wire-layout change.
 pub const HOST_CAP_CLIPBOARD: u8 = 0x02;
 
+/// [`Hello::client_caps`] bit: the client renders the host cursor LOCALLY
+/// (design/remote-desktop-sweep.md M2). It consumes [`CursorShape`](super::control::CursorShape)
+/// control messages (RGBA bitmap + hotspot, cached by serial) and per-frame
+/// [`CursorState`](super::datagram::CursorState) `0xD0` datagrams (position/visibility), and
+/// draws the pointer itself — so the host must STOP compositing the cursor into the video
+/// (`SessionPlan.cursor_blend = false`) or the user sees it twice. Active only when the host
+/// answers with [`HOST_CAP_CURSOR`] (capable-and-agreed, the 444/clipboard precedent); toward
+/// an older or incapable host nothing changes.
+pub const CLIENT_CAP_CURSOR: u8 = 0x01;
+
+/// [`Welcome::host_caps`] bit: the host CAN forward the cursor out-of-band (it captures cursor
+/// metadata separately from the frame — the Linux portal `SPA_META_Cursor` path; NOT gamescope,
+/// whose capture carries no cursor, and NOT Windows yet, where DWM composites into the IDD
+/// frame). Set only when the client asked via [`CLIENT_CAP_CURSOR`]; when both bits agree the
+/// host stops blending and ships [`CursorShape`](super::control::CursorShape) +
+/// [`CursorState`](super::datagram::CursorState) instead.
+pub const HOST_CAP_CURSOR: u8 = 0x04;
+
 /// [`Hello::video_codecs`] bit: the client can decode H.264 / AVC. The GPU-less **software**
 /// encode path (openh264) emits H.264, so a client that wants to stream from a software host MUST
 /// advertise this.