fix(audio): close loopback capture context on teardown (release tap worklet)

The loopback context was reused across engages (_lbCtx || new), but teardown
only stopped the stream + deactivated the tap — never closing the context or
detaching the worklet node. Each exclusive<->shared switch orphaned a live
tap worklet on the long-lived context. Use a fresh context per session and
close it on disengage. Adds a test asserting the context is closed on teardown.

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

static/app.js

@@ -5418,7 +5418,9 @@ window.jucePlayer = jucePlayer;
             throw new Error('no loopback audio track');
         }
         try {
-            _lbCtx = _lbCtx || new AudioContext();
+            // Fresh context per session (not reused) so teardown's close()
+            // fully releases the tap worklet node — see _teardownLoopback.
+            _lbCtx = new AudioContext();
             if (_lbCtx.state !== 'running') await _lbCtx.resume().catch(() => {});
             const source = _lbCtx.createMediaStreamSource(stream);
             const tap = _makeTap(_lbCtx);
@@ -5445,6 +5447,13 @@ window.jucePlayer = jucePlayer;
         if (_lbTap) _lbTap.active = false;
         if (_lbStream) for (const t of _lbStream.getTracks()) t.stop();
         _lbStream = null; _lbTap = null;
+        // Close the capture context so its tap worklet node is released. The
+        // context is per-session (not reused): without this, each exclusive⇄
+        // shared switch orphaned a live worklet on a long-lived context.
+        if (_lbCtx) {
+            try { await _lbCtx.close(); } catch (_) { /* already closed */ }
+            _lbCtx = null;
+        }
         if (_lbPageMuted && typeof api.setPageMuted === 'function') {
             try { await api.setPageMuted(false); } catch (_) { /* engine gone */ }
         }

tests/js/renderer_bus_feeder.test.js

@@ -54,6 +54,7 @@ function makeFakeContext(sampleRate = 48000) {
             this.mediaStreamSource = stream;
             return { connect() {}, disconnect() {} };
         },
+        close() { this.closed = true; return Promise.resolve(); },
     };
     return ctx;
 }
@@ -231,6 +232,22 @@ test('exclusive output + loopback available → engages without any song loaded'
     assert.equal(sb.__calls.setPageMuted.length, 0, 'suppress constraint honoured — no page mute');
 });
 
+test('loopback context is closed on disengage (no orphaned tap worklet)', async () => {
+    let excl = true;
+    const stream = makeLoopbackStream();
+    const sb = makeSandbox({ exclusive: () => excl, displayMedia: () => Promise.resolve(stream) });
+
+    await sb.window._reevaluateRendererBus();          // engage loopback
+    const lbCtx = sb.__createdContexts.at(-1);
+    assert.equal(lbCtx?.mediaStreamSource, stream, 'loopback engaged');
+    assert.notEqual(lbCtx.closed, true, 'context live while engaged');
+
+    excl = false;
+    await sb.window._reevaluateRendererBus();          // disengage
+    assert.equal(lbCtx.closed, true, 'loopback context closed on disengage');
+    assert.ok(stream.__stopped.includes('audio'), 'capture stream stopped');
+});
+
 test('loopback preferred over stems when both available', async () => {
     const stream = makeLoopbackStream();
     const sb = makeSandbox({ exclusive: () => true, displayMedia: () => Promise.resolve(stream) });