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>

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) });