fix(playback): the song queue must survive a playSong wrapper that drops options (#977)

Tester: "Passports does not advance in the song queue."

The play queue tells playSong "don't clear the queue I'm driving" by passing
options.fromQueue. But window.playSong is wrapped by a CHAIN of plugins —
nam_tone, midi_amp, fretboard, invert_highway, tabview — and each wrapper
forwards only (filename, arrangement), silently dropping the options object. So
fromQueue never reached playSong: it cleared the queue the instant its first
song started, and a gig/album/playlist never advanced.

Reproduced on the real build via a queue.start + a hooked clear(): the queue
went inactive with 0 remaining immediately after start, and the clear stack ran
through nam_tone -> midi_amp -> invert_highway -> fretboard -> session.js.

Fixing six plugin wrappers is whack-a-mole and the next plugin re-breaks it.
Fix it at the source instead: the queue raises an out-of-band flag
(_consumeInternalPlay, one-shot) beside the wrapper chain, not through it, and
playSong's clear-guard honours it. options.fromQueue stays as the in-band path.
The flag is consumed on read so a later MANUAL play still abandons the queue.

Verified on the real build: the gig queue stays active after start and advances
on song:ended (Iron Maiden -> Blind Guardian), and a manual play still clears.

Tests drive the real clear-guard against the queue for: a dropped-options
wrapper (the bug), the one-shot manual-play-still-clears invariant, and the
in-band fromQueue path on its own. All 3 fail on the pre-fix source. JS 1211/1211.

static/app.js

@@ -1334,12 +1334,25 @@ if (window.feedBack) window.feedBack.closeCurrentSong = closeCurrentSong;
 // leaving the player still leaves — and abandons the queue.
 window.feedBack.playQueue = (function () {
     let list = [], idx = -1, source = '', arrangements = null;
+    // Set true by _play() right before it drives playSong, consumed once by
+    // playSong's clear-guard. The primary "don't clear the queue I'm driving"
+    // signal is options.fromQueue, but a chain of plugin playSong wrappers
+    // (nam_tone, midi_amp, fretboard, invert_highway, tabview, ...) forward only
+    // (filename, arrangement) and silently drop the options object — so the flag
+    // never arrived and the queue cleared itself the instant its first song
+    // started (a gig/album/playlist never advanced). This flag rides beside the
+    // wrapper chain, not through it.
+    let _internalPlay = false;
     const active = () => idx >= 0 && idx < list.length;
     const hasNext = () => active() && idx < list.length - 1;
     function clear() { list = []; idx = -1; source = ''; arrangements = null; }
     function _play(i) {
         const fn = list[i];
-        // fromQueue keeps the queue from clearing itself; playSong decodeURIs.
+        // fromQueue is the in-band signal; _internalPlay is the out-of-band one
+        // that survives wrapper chains dropping the options arg. Both set; either
+        // suffices. playSong runs its clear-guard synchronously at entry, and the
+        // wrapper chain reaches it synchronously, so the flag is still set then.
+        _internalPlay = true;
         window.playSong(encodeURIComponent(fn), arrangements ? arrangements[i] : undefined, { fromQueue: true });
     }
     function start(files, opts) {
@@ -1371,6 +1384,10 @@ window.feedBack.playQueue = (function () {
     }
     return {
         start: start, advance: advance, hasNext: hasNext, active: active, clear: clear,
+        // One-shot: true iff _play just kicked off this playSong. Consumed on
+        // read so a later MANUAL play still clears the queue. playSong calls this
+        // instead of trusting options.fromQueue to survive the wrapper chain.
+        _consumeInternalPlay: function () { const v = _internalPlay; _internalPlay = false; return v; },
         source: function () { return source; },
         remaining: function () { return active() ? list.length - idx - 1 : 0; },
         // What's coming, for consumers that RENDER the queue (a results