feat(headless-kde): reliable bring-up — readiness probe, fix portal ordering/env (roadmap #1 phase 1)

Headless KDE startup was a chain of timing-sensitive handoffs gated by a blind `sleep 2`,
the dominant source of black screens. Phase-1 fixes:

- New `punktfunk-host probe-compositor` subcommand: exits 0 iff the detected compositor is
  up AND ready to create a virtual output now. KWin gets a real check (connect + registry
  roundtrip + the privileged zkde_screencast global must be advertised — what the backend
  needs); gamescope/Mutter/wlroots create on demand so the probe just confirms Linux.
  (vdisplay::probe dispatcher + kwin::probe; reuses kwin.rs's existing roundtrip path.)
- run-headless-kde.sh: replace `sleep 2` with an active readiness wait (poll probe-compositor
  until ready, 30s deadline, and bail with kwin's log if kwin_wayland exits during init).
  Move the portal restart to AFTER readiness, and precede it with `systemctl --user
  import-environment` + `dbus-update-activation-environment` (the missing env import — the
  Sway script does this; without it a restarted portal inherits a stale/empty WAYLAND_DISPLAY,
  which is the "streams but eats no input/audio" failure). kwin's stderr → a log file.

Validated: probe-compositor exits 0 "Kwin ready" against the live session, exit 1 with a
clear diagnostic when the compositor is absent. 114 tests green, clippy/fmt clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/vdisplay.rs

@@ -111,6 +111,30 @@ pub fn open(compositor: Compositor) -> Result<Box<dyn VirtualDisplay>> {
     }
 }
 
+/// Readiness probe for `compositor`: is it up and able to create a virtual output *right
+/// now*? A session-bringup script polls this (via `punktfunk-host probe-compositor`) to gate
+/// on actual readiness instead of racing the compositor with a blind sleep.
+///
+/// KWin gets a real check (the privileged `zkde_screencast` global must be advertised). The
+/// others are spawn/D-Bus/portal-based and have no equivalent pre-flight global, so a probe
+/// just confirms the backend opens — `Ok(())` means "go ahead and try `create`".
+pub fn probe(compositor: Compositor) -> Result<()> {
+    #[cfg(target_os = "linux")]
+    {
+        match compositor {
+            Compositor::Kwin => kwin::probe(),
+            // gamescope spawns its own nested session per `create`; Mutter is D-Bus on demand;
+            // wlroots creates the output on demand — nothing to pre-check beyond "Linux".
+            Compositor::Gamescope | Compositor::Mutter | Compositor::Wlroots => Ok(()),
+        }
+    }
+    #[cfg(not(target_os = "linux"))]
+    {
+        let _ = compositor;
+        anyhow::bail!("virtual displays require Linux (Wayland compositor)")
+    }
+}
+
 /// Path of the file where the gamescope backend relays the nested session's `LIBEI_SOCKET`
 /// (gamescope's EIS server) for the input injector.
 #[cfg(target_os = "linux")]