feat: elapsed + drift formatting helpers

src/core/format.js

@@ -0,0 +1,13 @@
+export function fmtElapsed(ms) {
+  const total = Math.floor(ms / 1000);
+  const h = Math.floor(total / 3600);
+  const m = Math.floor((total % 3600) / 60);
+  return `${h}:${String(m).padStart(2, '0')}`;
+}
+
+export function fmtDrift(driftMin) {
+  if (driftMin == null) return '';
+  const r = Math.round(driftMin);
+  if (r === 0) return 'on time';
+  return r > 0 ? `+${r} behind` : `${Math.abs(r)} ahead`;
+}

tests/core/format.test.js

@@ -0,0 +1,16 @@
+import { describe, it, expect } from 'vitest';
+import { fmtElapsed, fmtDrift } from '../../src/core/format.js';
+
+describe('formatting', () => {
+  it('formats elapsed time', () => {
+    expect(fmtElapsed(0)).toBe('0:00');
+    expect(fmtElapsed(65 * 1000)).toBe('0:01');
+    expect(fmtElapsed(112 * 60 * 1000)).toBe('1:52');
+  });
+  it('formats drift with direction', () => {
+    expect(fmtDrift(null)).toBe('');
+    expect(fmtDrift(0)).toBe('on time');
+    expect(fmtDrift(6)).toBe('+6 behind');
+    expect(fmtDrift(-3)).toBe('3 ahead');
+  });
+});