fix(vdisplay): Windows admission default is reject, not join (single-capturer limit)

Two concurrent Windows sessions both drive the same pf-vdisplay monitor's
single-capturer IDD-push channel (newest-delivery-wins), which freezes the live
client and can wedge the driver (observed live: a concurrent-session test wedged
.173 → Moonlight 'no video'; needed a reboot). True multi-session capture is §6.6/
Stage 7. So on Windows 'separate' (incl. the unconfigured default) now resolves to
REJECT — a 2nd client gets a clean 503 and the live session is protected — instead
of join (which would freeze it). join/steal stay explicit opt-ins; Linux keeps
separate (real multi-view). Centralized as admission::effective_conflict(), shared
by the native handshake + GameStream h_launch.

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

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

@@ -149,10 +149,9 @@ async fn h_launch(
             .unwrap()
             .as_ref()
             .map(|s| (s.owner_fp, (s.width, s.height, s.fps)));
-        let conflict = crate::vdisplay::policy::prefs()
-            .configured_effective()
-            .map(|e| e.mode_conflict)
-            .unwrap_or(crate::vdisplay::policy::ModeConflict::Separate);
+        // Same Windows default as the native path (separate → reject; see `effective_conflict`) so a
+        // 2nd Moonlight client gets a clean 503 rather than wedging the shared monitor's capture.
+        let conflict = crate::vdisplay::admission::effective_conflict();
         match gamestream_admission(live, req_fp, conflict) {
             GsDecision::Serve => {}
             GsDecision::Join((w, h, f)) => {