fix(venue): fly in once per set, not before every song (#978)

Tester, mid-gig: "the second song in the gig started when the first one ended.
But it showed the flyover intro again."

The flyover is arriving at the venue, and you arrive once. #968 stopped it
replaying on an arrangement SWITCH (same filename), but a gig's song 2 is a
genuinely different file, so it took the full-teardown path and played the
arrival flyover again — the camera flew in from the back of the room before
every track of the set.

The play queue now answers isContinuation(): false for the first song of a set
(or a standalone play — an arrival), true for song 2..N. onSongLoaded carries
the room over to the new song's loop on a continuation, and only a real arrival
plays the intro.

Verified on the built AppImage: isContinuation goes false (song 1) -> true
(song 2) across an advance, and song 2 no longer flies in.

Also confirmed NOT a bug, same session: "didn't show the author for the second
song." The credits card shows on a queue advance whenever the song carries
authors — reproduced with a song that has them as the advanced-to track. The
tester's song 2 simply had no `authors:` metadata (most auto-converted feedpaks
don't). No code change.

Tests: isContinuation across start/advance/clear, and that onSongLoaded gates the
flyover on the continuation check. Both fail on pre-fix source. JS 1214/1214.

static/v3/venue-crowd.js

@@ -529,7 +529,27 @@
         _loadingLoop = null;
         _fadingLoop = null;
         if (_venueActive && _manifest) {
-            if (!playIntro()) showLoop(machine.current, FADE_MS);
+            // The flyover is ARRIVING at the venue, and you arrive once. Songs
+            // 2..N of a set (a gig / album / playlist) are a NEW song but the
+            // SAME arrival — the camera should not fly in from the back of the
+            // room before every track (tester: "it showed the flyover intro
+            // again" on a gig's second song). Continue the room to the new song's
+            // loop; only a first-song / standalone arrival flies in.
+            if (_isSetContinuation()) showLoop(machine.current, FADE_MS);
+            else if (!playIntro()) showLoop(machine.current, FADE_MS);
+        }
+    }
+
+    // Is this song load a continuation of a play queue (a set already in
+    // progress), rather than an arrival? True for song 2..N of a gig/album/
+    // playlist. The queue owns the answer; treat any error / absent queue as
+    // "not a continuation" so a standalone play still flies in.
+    function _isSetContinuation() {
+        try {
+            const q = window.feedBack && window.feedBack.playQueue;
+            return !!(q && typeof q.isContinuation === 'function' && q.isContinuation());
+        } catch (_) {
+            return false;
         }
     }