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/Cargo.toml

@@ -30,6 +30,12 @@ pipewire = "0.9"
 libc = "0.2"
 # ashpd 0.13 uses the tokio runtime for the one-time portal handshake (control plane).
 tokio = { version = "1", features = ["rt", "rt-multi-thread", "net", "time"] }
+# XFixes cursor source for gamescope (remote-desktop-sweep Phase C): gamescope paints no
+# `SPA_META_Cursor`, so the pointer never reaches the PipeWire node. We read the shape/hotspot/
+# visibility from gamescope's nested Xwayland via XFixes instead and feed the existing cursor slot.
+# `RustConnection` is the pure-Rust default (no libxcb link → no new C dependency on the host); the
+# `xfixes` feature (auto-pulls `render` + `shape`) is what exposes GetCursorImage/SelectCursorInput.
+x11rb = { version = "0.13", default-features = false, features = ["xfixes"] }
 
 [target.'cfg(target_os = "windows")'.dependencies]
 # The host<->driver wire contract for the sealed frame channel (control IOCTL structs + frame header).