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>

scripts/headless/run-headless-kde.sh

@@ -1,12 +1,11 @@
 #!/usr/bin/env bash
 # Headless KDE Plasma session for the punktfunk host (no KMS scanout → kwin --virtual).
 #
-# Brings up the full desktop, not just the compositor. The env matters: without
+# Brings up the full desktop, not just the compositor, and waits for it to be *actually
+# ready* before starting the portals/plasma — no blind `sleep`. The env matters: without
 # XDG_MENU_PREFIX=plasma- the launcher resolves ${XDG_MENU_PREFIX}applications.menu →
 # "applications.menu", which doesn't exist on KDE installs (it ships
-# plasma-applications.menu) — plasmashell runs fine but the menu shows NO applications
-# and no System Settings entry. kded6/krunner/kglobalacceld are D-Bus-activated once
-# plasmashell starts in the right session env.
+# plasma-applications.menu) — plasmashell runs fine but the menu shows NO applications.
 #
 #   bash scripts/headless/run-headless-kde.sh [WxH]    # default 1920x1080
 #
@@ -29,18 +28,54 @@ export DESKTOP_SESSION=plasma
 export WAYLAND_DISPLAY=wayland-kde
 export KWIN_WAYLAND_NO_PERMISSION_CHECKS=1
 
-kwin_wayland --virtual --width "$W" --height "$H" --no-lockscreen \
-    --socket "$WAYLAND_DISPLAY" &
-KWIN_PID=$!
-sleep 2
+# The probe binary (gates readiness on KWin actually exposing zkde_screencast — not merely
+# the socket existing). Use a release build if present, else fall back to `cargo run`.
+ROOT="$(cd "$(dirname "$0")/../.." && pwd)"
+if [[ -x "$ROOT/target/release/punktfunk-host" ]]; then
+    PROBE=("$ROOT/target/release/punktfunk-host" probe-compositor)
+elif [[ -x "$ROOT/target/debug/punktfunk-host" ]]; then
+    PROBE=("$ROOT/target/debug/punktfunk-host" probe-compositor)
+else
+    PROBE=(cargo run -q --manifest-path "$ROOT/Cargo.toml" -p punktfunk-host -- probe-compositor)
+fi
 
-# The xdg-desktop-portal processes bind to the compositor that existed when THEY started;
-# after a kwin restart the stale instances point at a dead socket and RemoteDesktop/EIS
-# (mouse/keyboard injection) times out. Restart them against the fresh compositor.
-systemctl --user try-restart plasma-xdg-desktop-portal-kde.service xdg-desktop-portal.service 2>/dev/null || true
+# kwin to its own log (so its EGL/GPU-init errors are captured, not lost to the terminal).
+KWIN_LOG="${TMPDIR:-/tmp}/punktfunk-kwin.log"
+kwin_wayland --virtual --width "$W" --height "$H" --no-lockscreen \
+    --socket "$WAYLAND_DISPLAY" >"$KWIN_LOG" 2>&1 &
+KWIN_PID=$!
+
+# Active readiness wait: poll until KWin is up AND advertises the zkde_screencast global
+# (what the virtual-output backend needs), or fail fast with a useful message. kwin can also
+# exit immediately if EGL/GPU init fails — catch that.
+echo "waiting for KWin ($RES) to become ready…"
+DEADLINE=$(( SECONDS + 30 ))
+until "${PROBE[@]}" >/dev/null 2>&1; do
+    if ! kill -0 "$KWIN_PID" 2>/dev/null; then
+        echo "ERROR: kwin_wayland exited during startup — see $KWIN_LOG:" >&2
+        tail -n 20 "$KWIN_LOG" >&2 || true
+        exit 1
+    fi
+    if (( SECONDS >= DEADLINE )); then
+        echo "ERROR: KWin did not become ready within 30s. Last probe:" >&2
+        "${PROBE[@]}" >&2 || true
+        exit 1
+    fi
+    sleep 0.5
+done
+echo "KWin ready."
+
+# Only NOW restart the portals, and against the correct env: the xdg-desktop-portal chain
+# binds the compositor that existed when it started, so a stale portal points at a dead
+# socket and RemoteDesktop/EIS (input injection) times out. Import the session env into the
+# systemd/D-Bus activation environment FIRST (the missing piece — the Sway script does this;
+# without it the restarted portal can inherit an empty WAYLAND_DISPLAY), then restart.
+systemctl --user import-environment WAYLAND_DISPLAY XDG_CURRENT_DESKTOP DBUS_SESSION_BUS_ADDRESS XDG_RUNTIME_DIR 2>/dev/null || true
+dbus-update-activation-environment --systemd WAYLAND_DISPLAY XDG_CURRENT_DESKTOP DBUS_SESSION_BUS_ADDRESS 2>/dev/null || true
+systemctl --user try-restart plasma-xdg-desktop-portal-kde.service xdg-desktop-portal-kde.service xdg-desktop-portal.service 2>/dev/null || true
 
 kbuildsycoca6 >/dev/null 2>&1 || true # rebuild the menu cache under the correct env
 plasmashell &
 
-echo "headless KDE up on $WAYLAND_DISPLAY ($RES), kwin pid $KWIN_PID"
+echo "headless KDE up on $WAYLAND_DISPLAY ($RES), kwin pid $KWIN_PID (log: $KWIN_LOG)"
 wait "$KWIN_PID"