refactor(youtube): extract subtitle generation pipeline

launcher/youtube/srt.test.ts

@@ -0,0 +1,32 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+
+import { parseSrt, stringifySrt } from './srt';
+
+test('parseSrt reads cue numbering timing and text', () => {
+  const cues = parseSrt(`1
+00:00:01,000 --> 00:00:02,000
+こんにちは
+
+2
+00:00:02,500 --> 00:00:03,000
+世界
+`);
+
+  assert.equal(cues.length, 2);
+  assert.equal(cues[0]?.start, '00:00:01,000');
+  assert.equal(cues[0]?.end, '00:00:02,000');
+  assert.equal(cues[0]?.text, 'こんにちは');
+  assert.equal(cues[1]?.text, '世界');
+});
+
+test('stringifySrt preserves parseable cue structure', () => {
+  const roundTrip = stringifySrt(
+    parseSrt(`1
+00:00:01,000 --> 00:00:02,000
+こんにちは
+`),
+  );
+
+  assert.match(roundTrip, /1\n00:00:01,000 --> 00:00:02,000\nこんにちは/);
+});