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>

include/punktfunk_core.h

@@ -977,6 +977,13 @@
 // The client's wire (protocol) version does not match the host's — one side needs updating.
 #define WIRE_VERSION_CLOSE_CODE 103
 
+// The host admitted the connection but could not stand the stream session up (compositor /
+// capture / encoder setup failed host-side). The close reason bytes carry the specific error
+// text for logs/diagnostics; clients render a stable "host-side failure" sentence. Before this
+// code, a setup failure reached the client as a bare dropped connection ("control stream
+// finished mid-frame") — indistinguishable from transport trouble.
+#define SETUP_FAILED_CLOSE_CODE 104
+
 // Minimum supported multiplier (renders under native, upscaled on present).
 #define MIN_SCALE 0.5
 
@@ -1010,6 +1017,7 @@ enum PunktfunkStatus
     PUNKTFUNK_STATUS_REJECTED_SUPERSEDED = -26,
     PUNKTFUNK_STATUS_REJECTED_WIRE_VERSION = -27,
     PUNKTFUNK_STATUS_REJECTED_BUSY = -28,
+    PUNKTFUNK_STATUS_REJECTED_SETUP_FAILED = -29,
     PUNKTFUNK_STATUS_PANIC = -99,
 };
 #ifndef __cplusplus