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/main.rs

@@ -27,7 +27,7 @@ mod vdisplay;
 #[cfg(target_os = "linux")]
 mod zerocopy;
 
-use anyhow::{bail, Result};
+use anyhow::{bail, Context, Result};
 use encode::Codec;
 use m0::{Options, Source};
 use std::path::PathBuf;
@@ -68,6 +68,15 @@ fn real_main() -> Result<()> {
         // Zero-copy FFI/GPU probe: init the EGL importer + CUDA context (no capture needed).
         #[cfg(target_os = "linux")]
         Some("zerocopy-probe") => zerocopy::probe(),
+        // Compositor readiness probe: exit 0 iff the (detected or PUNKTFUNK_COMPOSITOR-forced)
+        // compositor is up and able to create a virtual output *now*. A session-bringup
+        // script polls this to gate on real readiness instead of a blind `sleep`.
+        Some("probe-compositor") => {
+            let compositor = vdisplay::detect()?;
+            vdisplay::probe(compositor).with_context(|| format!("{compositor:?} not ready"))?;
+            println!("{compositor:?} ready");
+            Ok(())
+        }
         // M0 pipeline spike.
         Some("m0") => m0::run(parse_m0(&args[1..])?),
         // M3: native punktfunk/1 host (QUIC control plane + UDP data plane).
@@ -308,6 +317,7 @@ USAGE:
                                   + the management REST API
     punktfunk-host openapi            print the management API's OpenAPI document (codegen)
     punktfunk-host m3-host [OPTIONS]  native punktfunk/1 host (QUIC control plane + UDP data plane)
+    punktfunk-host probe-compositor   exit 0 iff the compositor is up + ready (session-bringup gate)
     punktfunk-host m0 [OPTIONS]       M0 capture→encode→file pipeline spike
 
 SERVE OPTIONS:

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")]

crates/punktfunk-host/src/vdisplay/kwin.rs

@@ -223,6 +223,29 @@ fn virtual_output_thread(
     }
 }
 
+/// Readiness probe: connect to the KWin Wayland socket, roundtrip the registry, and confirm
+/// the privileged `zkde_screencast` global is actually advertised. This is exactly what
+/// [`run`] needs before it can create a virtual output, so a session-bringup script can poll
+/// this to gate on the compositor being *ready* (not merely the socket existing) instead of
+/// racing it with a blind sleep. `Ok(())` = ready; `Err` = not ready / no global yet.
+pub fn probe() -> Result<()> {
+    let conn = Connection::connect_to_env()
+        .context("connect to KWin Wayland (is WAYLAND_DISPLAY set to the KWin socket?)")?;
+    let mut queue = conn.new_event_queue();
+    let qh = queue.handle();
+    let _registry = conn.display().get_registry(&qh, ());
+    let mut state = State::default();
+    queue.roundtrip(&mut state).context("registry roundtrip")?;
+    if state.screencast.is_none() {
+        bail!(
+            "KWin is up but does not (yet) expose zkde_screencast_unstable_v1 — needs a real \
+             KDE session (or KWIN_WAYLAND_NO_PERMISSION_CHECKS=1), and KWin ≥ 6.5.6 for the \
+             headless virtual output"
+        );
+    }
+    Ok(())
+}
+
 fn run(
     width: u32,
     height: u32,