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/mgmt.rs

@@ -2498,6 +2498,7 @@ mod tests {
             fps: 120,
             appid: 1,
             peer_ip: None,
+            owner_fp: None,
         });
         state.streaming.store(true, Ordering::SeqCst);
 
@@ -2624,6 +2625,7 @@ mod tests {
             fps: 60,
             appid: 1,
             peer_ip: None,
+            owner_fp: None,
         });
 
         let del = axum::http::Request::delete("/api/v1/session")