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-vdisplay/src/vdisplay/backend.rs

@@ -142,13 +142,25 @@ pub trait VirtualDisplay: Send {
     /// the backend's default EDID. Default: no-op — only the Windows pf-vdisplay backend can mint
     /// per-monitor EDIDs today (the Linux compositors' virtual outputs take no EDID from us).
     fn set_client_hdr(&mut self, _hdr: Option<punktfunk_core::quic::HdrMeta>) {}
-    /// Ask the backend for an out-of-band HARDWARE CURSOR on the created output (the M2c cursor
-    /// channel): the compositor/OS stops compositing the pointer into captured frames and the
-    /// capture layer surfaces shape/position separately. Carried on the backend instance; set
-    /// once before [`create`](Self::create). Default: no-op — only the Windows pf-vdisplay
-    /// backend (IddCx hardware cursor, driver proto v5) implements it; the Linux portal path
-    /// gets the same split from `SPA_META_Cursor` without asking.
+    /// Ask the backend for an OUT-OF-BAND cursor on the created output (the cursor channel):
+    /// the compositor/OS stops compositing the pointer into captured frames and the capture
+    /// layer surfaces shape/position separately. Carried on the backend instance; set once
+    /// before [`create`](Self::create) (both session paths pass `cursor_forward`). Off = the
+    /// compositor EMBEDS the pointer into frames — zero host-side cursor work, the pre-channel
+    /// path — which is what every session without the negotiated cursor cap gets (Moonlight /
+    /// GameStream / legacy clients / capture-mode starts), mirroring the Windows no-regression
+    /// gate. Implementations: Windows pf-vdisplay (IddCx hardware cursor, driver proto v5);
+    /// KWin (zkde `pointer` metadata vs embedded); Mutter (`cursor-mode` metadata vs embedded);
+    /// wlroots/hyprland (portal `CursorMode`). Default: no-op (gamescope has no cursor either
+    /// way — see the Phase C source).
     fn set_hw_cursor(&mut self, _on: bool) {}
+    /// The out-of-band-cursor request currently set (see [`set_hw_cursor`](Self::set_hw_cursor)).
+    /// The registry includes it in the keep-alive REUSE key: a kept embedded-pointer display can
+    /// never serve a cursor-channel session (its stream has no cursor metadata to forward) nor
+    /// vice versa (the pointer would be missing from frames).
+    fn hw_cursor(&self) -> bool {
+        false
+    }
     /// The stable identity slot the backend resolved for the most recent [`create`](Self::create) —
     /// the per-client id the identity policy assigned (`Some`), or `None` for shared/anonymous. The
     /// registry reads it right after `create` to key the display's group **arrangement** (manual