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/punktfunk-core/src/reject.rs

@@ -33,6 +33,12 @@ pub const PAIR_APPROVAL_TIMEOUT_CLOSE_CODE: u32 = 0x65;
 pub const PAIR_SUPERSEDED_CLOSE_CODE: u32 = 0x66;
 /// The client's wire (protocol) version does not match the host's — one side needs updating.
 pub const WIRE_VERSION_CLOSE_CODE: u32 = 0x67;
+/// 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.
+pub const SETUP_FAILED_CLOSE_CODE: u32 = 0x68;
 
 /// Why a host turned a connection away, decoded from the QUIC application close code — the
 /// client-side view of [`PAIR_NOT_ARMED_CLOSE_CODE`]..[`WIRE_VERSION_CLOSE_CODE`] plus
@@ -59,6 +65,9 @@ pub enum RejectReason {
     WireVersionMismatch,
     /// The host refused admission because a conflicting session is live.
     Busy,
+    /// The host admitted the connection but failed to start the stream session (host-side
+    /// setup error — the host log has the specific cause).
+    SetupFailed,
 }
 
 impl RejectReason {
@@ -75,6 +84,7 @@ impl RejectReason {
             PAIR_SUPERSEDED_CLOSE_CODE => Self::Superseded,
             WIRE_VERSION_CLOSE_CODE => Self::WireVersionMismatch,
             REJECT_BUSY_CLOSE_CODE => Self::Busy,
+            SETUP_FAILED_CLOSE_CODE => Self::SetupFailed,
             _ => return None,
         })
     }
@@ -91,6 +101,7 @@ impl RejectReason {
             Self::Superseded => PAIR_SUPERSEDED_CLOSE_CODE,
             Self::WireVersionMismatch => WIRE_VERSION_CLOSE_CODE,
             Self::Busy => REJECT_BUSY_CLOSE_CODE,
+            Self::SetupFailed => SETUP_FAILED_CLOSE_CODE,
         }
     }
 
@@ -107,6 +118,7 @@ impl RejectReason {
             Self::Superseded => "superseded",
             Self::WireVersionMismatch => "wire-version",
             Self::Busy => "busy",
+            Self::SetupFailed => "setup-failed",
         }
     }
 }
@@ -125,6 +137,7 @@ impl std::fmt::Display for RejectReason {
             Self::Superseded => "a newer request from this device replaced this one",
             Self::WireVersionMismatch => "client and host versions do not match",
             Self::Busy => "the host is busy with another session",
+            Self::SetupFailed => "the host could not start the stream session",
         })
     }
 }