feat(capture): gamescope cursor via XFixes shape + QueryPointer position (Phase C)

gamescope excludes its pointer from the PipeWire node it feeds us and can't embed
one either (`set_hw_cursor` is inert), so every gamescope stream was cursorless.
Read the pointer from gamescope's nested Xwayland instead — XFixesGetCursorImage
for shape/hotspot/visibility, core QueryPointer for position — and publish a
CursorOverlay into the capturer's existing `cursor_live` slot, so the encoder
blend composites it into the video exactly like the SPA_META_Cursor path.

- pf-capture/src/linux/xfixes_cursor.rs (new): the XFixes reader. Connects to
  EVERY nested Xwayland (Gaming Mode runs one per --xwayland-count) and follows
  the focused one each tick (the display whose pointer moves), reading that
  display's own cursor shape. Un-premultiplies ARGB -> straight RGBA. Drop stops
  the thread.
- Capturer::attach_gamescope_cursor + the PortalCapturer override spawn it into
  the same `cursor_live` slot; pf_vdisplay::gamescope_xwayland_cursor_targets
  discovers the (DISPLAY, XAUTHORITY) pairs via the GAMESCOPE_WAYLAND_DISPLAY scan.
- host: SessionPlan.gamescope_cursor (set from the compositor at both resolve
  sites AND on a mid-stream Desktop->Gaming switch); the blend gate now builds the
  encoder blend for gamescope; a sibling composite arm attaches capturer.cursor()
  per tick for capture-mode clients (no channel needed). native NV12 is disabled
  for these sessions — that encode path can't blend the cursor (it assumes
  gamescope embeds the pointer), so we capture RGB and route to the proven CUDA
  VkSlotBlend / compute-CSC blend.
- the pipewire thread no longer clobbers `cursor_live` with None on a buffer that
  carries no SPA_META_Cursor (gamescope) — that raced the XFixes writer and
  strobed the composited pointer on/off.

On-glass (home-bazzite-2, RTX 5070 Ti, Gaming Mode): cursor visible + steady in
Big Picture and CS2 menus, follows focus between the two Xwaylands, hidden in-game.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/pf-capture/src/lib.rs

@@ -88,6 +88,15 @@ pub trait Capturer: Send {
     /// the encode loop blends its overlay instead.
     fn set_cursor_forward(&mut self, _on: bool) {}
 
+    /// Attach a gamescope cursor source (remote-desktop-sweep Phase C). gamescope paints no
+    /// `SPA_META_Cursor`, so [`cursor`](Self::cursor)'s slot stays empty — this hands the Linux
+    /// portal capturer gamescope's nested Xwayland `(DISPLAY, XAUTHORITY)` targets (it may run
+    /// several — one per `--xwayland-count`) so it reads the pointer shape/position over X11
+    /// (XFixes + QueryPointer), following whichever display is focused, and publishes it into that
+    /// same slot. Called once, after the capturer is built, only for gamescope sessions. Default
+    /// no-op: every non-gamescope capturer already has a cursor source.
+    fn attach_gamescope_cursor(&mut self, _targets: Vec<(String, Option<String>)>) {}
+
     fn hdr_meta(&self) -> Option<punktfunk_core::quic::HdrMeta> {
         None
     }