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-vdisplay/src/vdisplay/routing.rs

@@ -162,6 +162,15 @@ pub fn launch_into_gamescope_session(cmd: &str) -> Result<std::process::Child> {
     gamescope::launch_into_session(cmd)
 }
 
+/// Every nested Xwayland `(DISPLAY, XAUTHORITY)` of the running gamescope session for the XFixes
+/// cursor source (remote-desktop-sweep Phase C) — gamescope can run several, and the pointer is on
+/// whichever is focused. Empty when no gamescope session is running / it exposes no Xwayland (the
+/// host then leaves gamescope cursorless, today's behaviour).
+#[cfg(target_os = "linux")]
+pub fn gamescope_xwayland_cursor_targets() -> Vec<(String, Option<String>)> {
+    gamescope::xwayland_cursor_targets()
+}
+
 /// B2: has a **dedicated** gamescope game session's game exited (its `node_id` doesn't reappear within a
 /// short window after capture loss)? The dedicated-spawn session ends cleanly on `true` instead of the
 /// capture-loss rebuild. Scoped to the session's OWN node so a coexisting gamescope doesn't mask the