feat(vdisplay): finish Stage 4 — typed reject, Windows join-default, GameStream 503

Completes the mode-conflict admission surface deferred from the initial Stage 4:

- REJECT now delivers the reason to the client: punktfunk/1 closes the QUIC
  connection with a distinct BUSY code (0x42) + the 'host busy: streaming WxH@Hz to
  <client>' string, which the client reads from ApplicationClosed (validated on
  loopback: the probe logs 'closed by peer: host busy … (code 66)').
- Windows default: separate (incl. the unconfigured default) resolves to JOIN — the
  Windows native host admits a second client at the live mode instead of the old
  silent last-wins reconfigure of the shared monitor (release-note behavior fix; the
  reconfigure is now opt-in as steal). separate stays multi-view on Linux.
- GameStream 503: h_launch tracks the session owner fp (LaunchSession.owner_fp, kept
  [u8;32] for Copy) and applies the policy when a DIFFERENT paired client launches —
  reject → 503 (Moonlight 'host busy'), join → serve the live mode, steal/separate →
  take over. Same-client re-launch is never a conflict.

Native reject-reason loopback-validated; Windows join-default pending .173 rebuild;
GameStream 503 pending a Moonlight client (can't drive /launch autonomously).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/gamestream/mod.rs

@@ -108,6 +108,11 @@ pub struct LaunchSession {
     /// unpaired RTSP peer cannot ride a paired client's launch (security-review 2026-06-28 #4).
     /// `None` if the address could not be captured (then RTSP falls back to launch-present only).
     pub peer_ip: Option<std::net::IpAddr>,
+    /// SHA-256 cert fingerprint of the paired client that owns this session — mode-conflict admission
+    /// (Stage 4) compares it against a launching client to tell a same-client re-launch (always
+    /// allowed) from a DIFFERENT client (subject to the `mode_conflict` policy). `[u8; 32]` keeps
+    /// [`LaunchSession`] `Copy`; `None` when the peer cert couldn't be read.
+    pub owner_fp: Option<[u8; 32]>,
 }
 
 /// Shared control-plane state used as the axum app state.