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.

tests/js/play_queue_peek.test.js

@@ -109,3 +109,20 @@ test('fromQueue in options still works on its own (in-band path)', () => {
     clearGuard(win, { fromQueue: true });
     assert.strictEqual(q.active(), true, 'options.fromQueue alone must still keep the queue');
 });
+
+// isContinuation(): true for song 2..N of a set, false for the first song / a
+// standalone play. The venue uses it to fly in once on arrival, then carry the
+// room between songs instead of replaying the arrival flyover every track
+// (tester: "it showed the flyover intro again" on a gig's second song).
+test('isContinuation is false on the first song, true after advancing', () => {
+    const { q } = makeQueue();
+    assert.strictEqual(q.isContinuation(), false, 'idle queue is not a continuation');
+    q.start(['a.sloppak', 'b.sloppak', 'c.sloppak'], { source: 'gig' });
+    assert.strictEqual(q.isContinuation(), false, 'the FIRST song of a set is an arrival, not a continuation');
+    q.advance();
+    assert.strictEqual(q.isContinuation(), true, 'song 2 is a continuation — no re-flyover');
+    q.advance();
+    assert.strictEqual(q.isContinuation(), true, 'song 3 too');
+    q.clear();
+    assert.strictEqual(q.isContinuation(), false, 'a cleared queue is not a continuation');
+});