feat(vdisplay): compositor-embedded pointer for sessions without the cursor channel (Phase B)

Since the cursor-channel work, every Linux virtual output was created in
metadata pointer mode — making ALL sessions depend on host-side cursor
compositing, including the ones that can never use the channel (Moonlight/
GameStream, legacy clients, capture-mode starts). Those sessions paid the
blend bring-up per session and, whenever a visible cursor was composited,
the loss of NVENC's stream-ordered submit — for a strictly worse cursor
than the compositor's own.

Mirror the Windows no-regression gate: the already-wired per-session
set_hw_cursor(cursor_forward) now drives the Linux backends too. A
cursor-channel session gets metadata (shapes forwarded, composite flip
blends host-side — today's validated path, unchanged); every other session
gets the pointer compositor-EMBEDDED at creation (KWin zkde pointer=2,
Mutter cursor-mode=1, wlroots/hyprland portal CursorMode::Embedded — their
pre-channel default; the portal pair also gains the Metadata arm for
channel sessions, wired but untested on-glass). SessionPlan.cursor_blend
narrows to cursor-forward sessions and rides into the direct-SDK NVENC
open, which skips the Vulkan slot-blend bring-up entirely when off —
embedded sessions ring on plain CUDA surfaces and pay zero cursor cost,
per-session or per-frame.

The keep-alive registry's reuse key grows the created pointer mode (new
VirtualDisplay::hw_cursor getter): a kept embedded display has no cursor
metadata for a channel session to forward, and a kept metadata display
would leave a channel-less session with no pointer in its frames.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/pf-encode/src/lib.rs

@@ -270,7 +270,7 @@ fn open_video_backend(
     chroma: ChromaFormat,
     cursor_blend: bool,
 ) -> Result<(Box<dyn Encoder>, &'static str)> {
-    let _ = cursor_blend; // consumed only by the Linux vulkan-encode arm below
+    let _ = cursor_blend; // consumed only by the Linux vulkan-encode + direct-NVENC arms below
     validate_dimensions(codec, width, height)?;
     // Refresh/fps must be positive and sane: fps feeds the encoder time_base (`Rational(1, fps)`)
     // and the pts→ns conversion (`pts * 1e9 / fps`), so 0 builds a 1/0 rational / divides by zero.
@@ -394,6 +394,7 @@ fn open_video_backend(
                 cuda,
                 bit_depth,
                 chroma,
+                cursor_blend,
             )
             .map(|e| (e, "nvenc"))
         };
@@ -729,12 +730,15 @@ fn open_nvenc_probed(
     cuda: bool,
     bit_depth: u8,
     chroma: ChromaFormat,
+    cursor_blend: bool,
 ) -> Result<Box<dyn Encoder>> {
-    // Direct-SDK NVENC (design/linux-direct-nvenc.md): the DEFAULT on NVIDIA, and only for a CUDA
-    // capture payload (it registers CUDADEVICEPTR inputs — a CPU/dmabuf frame can't feed it, so those
-    // keep the libav path; and `cuda` is false on AMD/Intel, so they stay on VAAPI). Set
-    // PUNKTFUNK_NVENC_DIRECT=0 to fall back to libav. It self-clamps the bitrate internally (its own
-    // level-ceiling binary search at session open), so it skips the probe-loop stepping below.
+    #[cfg(not(feature = "nvenc"))]
+    let _ = cursor_blend; // consumed by the direct-SDK arm below
+                          // Direct-SDK NVENC (design/linux-direct-nvenc.md): the DEFAULT on NVIDIA, and only for a CUDA
+                          // capture payload (it registers CUDADEVICEPTR inputs — a CPU/dmabuf frame can't feed it, so those
+                          // keep the libav path; and `cuda` is false on AMD/Intel, so they stay on VAAPI). Set
+                          // PUNKTFUNK_NVENC_DIRECT=0 to fall back to libav. It self-clamps the bitrate internally (its own
+                          // level-ceiling binary search at session open), so it skips the probe-loop stepping below.
     #[cfg(feature = "nvenc")]
     if cuda && nvenc_direct_enabled() {
         tracing::info!(
@@ -751,6 +755,7 @@ fn open_nvenc_probed(
             cuda,
             bit_depth,
             chroma,
+            cursor_blend,
         )?) as Box<dyn Encoder>);
     }
     // The silent-degrade trap: a build without `--features nvenc` compiles the direct-SDK