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>

Cargo.lock

@@ -1459,6 +1459,16 @@ dependencies = [
  "version_check",
 ]
 
+[[package]]
+name = "gethostname"
+version = "1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1bd49230192a3797a9a4d6abe9b3eed6f7fa4c8a8a4947977c6f80025f92cbd8"
+dependencies = [
+ "rustix",
+ "windows-link 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
 [[package]]
 name = "getrandom"
 version = "0.2.17"
@@ -2829,6 +2839,7 @@ dependencies = [
  "tokio",
  "tracing",
  "windows 0.62.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "x11rb",
 ]
 
 [[package]]
@@ -5886,6 +5897,23 @@ version = "0.6.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1ffae5123b2d3fc086436f8834ae3ab053a283cfac8fe0a0b8eaae044768a4c4"
 
+[[package]]
+name = "x11rb"
+version = "0.13.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9993aa5be5a26815fe2c3eacfc1fde061fc1a1f094bf1ad2a18bf9c495dd7414"
+dependencies = [
+ "gethostname",
+ "rustix",
+ "x11rb-protocol",
+]
+
+[[package]]
+name = "x11rb-protocol"
+version = "0.13.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ea6fc2961e4ef194dcbfe56bb845534d0dc8098940c7e5c012a258bfec6701bd"
+
 [[package]]
 name = "x509-parser"
 version = "0.16.0"