fix(host/linux): survive and self-heal dead gamescope sessions on headless SteamOS boxes

Three fixes for the "most connects simply fail" trap on a SteamOS host
with no physical display (VM testbox, panel-less mini PC):

- restore path: skip the restore-to-physical-panel when no DRM connector
  reports a connected display — removing the headless drop-in and
  restarting gamescope-session.target on such a box just crash-loops
  gamescope and strands every later connect on "no usable compositor".
  Keep the headless session (and the takeover state) instead.
- connect path: when no live graphical session exists but the MANAGED
  gamescope infra is present (SteamOS gamescope-session / Bazzite
  session-plus), route to the gamescope takeover — which rebuilds the
  session at the client's mode — instead of failing the connect.
- error delivery: a session-setup failure used to just drop the QUIC
  connection, reaching the client as "control stream finished mid-frame"
  (indistinguishable from transport trouble). Close with a typed
  setup-failed code (0x68, RejectedSetupFailed = -29) carrying the error
  text, and give the client handshake/pairing error paths a short grace
  for the CONNECTION_CLOSE to beat the stream error it caused.

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

crates/pf-vdisplay/src/vdisplay/routing.rs

@@ -207,6 +207,20 @@ pub fn cancel_pending_tv_restore() {
 #[cfg(not(target_os = "linux"))]
 pub fn cancel_pending_tv_restore() {}
 
+/// Can the MANAGED gamescope path stand a session up from nothing on this box (SteamOS's
+/// `gamescope-session` launcher or Bazzite's `gamescope-session-plus` present)? Lets the connect
+/// path route a "no live graphical session" box to the gamescope takeover — which rebuilds the
+/// session at the client's mode — instead of failing the connect. Always `false` off Linux.
+#[cfg(target_os = "linux")]
+pub fn managed_session_available() -> bool {
+    gamescope::managed_session_available()
+}
+
+#[cfg(not(target_os = "linux"))]
+pub fn managed_session_available() -> bool {
+    false
+}
+
 /// Call when a client session ends: if the host-managed gamescope path took over a box's autologin
 /// gaming session (stopped its single-instance Steam to stream at the client's mode), **schedule** a
 /// debounced restore so the TV returns to gaming mode — unless a client reconnects within the window