Add lazy piano transcription and optional leased server worker

frontend/piano-core.test.js

@@ -0,0 +1,50 @@
+const { test } = require('node:test'),
+  assert = require('node:assert/strict');
+const P = require('./piano-core');
+test('compact notes sort, round timing and infer hands below middle C', () => {
+  const notes = P.normalizeNotes([
+    { pitch: 60, start: 1.23456, end: 1.8, velocity: 85 },
+    { pitch: 48, start: 0, end: 0.5, velocity: 150 },
+    { pitch: 55, start: 2, end: 3, velocity: 40, hand: 'right' },
+  ]);
+  assert.equal(notes[0].hand, 'left');
+  assert.equal(notes[0].velocity, 127);
+  assert.equal(notes[1].start, 1.235);
+  assert.equal(notes[1].hand, 'right');
+  assert.equal(notes[2].hand, 'right');
+});
+test('invalid notes are rejected and tiny notes retain positive duration', () => {
+  assert.throws(() => P.normalizeNotes([{ pitch: 200, start: 0, end: 1, velocity: 90 }]));
+  assert.throws(() => P.normalizeNotes([{ pitch: 60, start: 2, end: 1, velocity: 90 }]));
+  const [note] = P.normalizeNotes([{ pitch: 60, start: 0.0001, end: 0.0002, velocity: 0 }]);
+  assert.ok(note.end > note.start);
+});
+test('Basic Pitch output converts amplitude to MIDI velocity and active notes use exclusive ends', () => {
+  const notes = P.fromBasicPitch([
+    { pitchMidi: 60, startTimeSeconds: 1, durationSeconds: 0.5, amplitude: 0.5 },
+  ]);
+  assert.equal(notes[0].velocity, 64);
+  assert.equal(P.activeNotes(notes, 1).length, 1);
+  assert.equal(P.activeNotes(notes, 1.5).length, 0);
+  assert.deepEqual(P.practiceRange(95, 100), { start: 95, end: 100 });
+  assert.equal(P.practiceRange(100, 100), null);
+});
+test('browser engine prefers WebGPU and reliably falls back to CPU', async () => {
+  const { selectBackend } = await import('./piano-engine.mjs');
+  const calls = [];
+  const tf = {
+    setBackend: async (name) => {
+      calls.push(name);
+      return true;
+    },
+  };
+  assert.equal(await selectBackend(tf, true, async () => {}), 'webgpu');
+  assert.deepEqual(calls, ['webgpu']);
+  const fallback = { setBackend: async (name) => name === 'cpu' };
+  assert.equal(
+    await selectBackend(fallback, true, async () => {
+      throw new Error('Unavailable GPU');
+    }),
+    'cpu',
+  );
+});