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/lib.rs

@@ -72,8 +72,8 @@ pub use session::{session_epoch, try_recover_session};
 #[path = "vdisplay/routing.rs"]
 pub(crate) mod routing;
 pub use routing::{
-    apply_input_env, restore_managed_session, restore_takeover_on_startup, start_restore_worker,
-    wants_dedicated_game_session,
+    apply_input_env, managed_session_available, restore_managed_session,
+    restore_takeover_on_startup, start_restore_worker, wants_dedicated_game_session,
 };
 #[cfg(target_os = "linux")]
 pub use routing::{