Implements src/transcript.rs: lenient JSONL parsing, message.id dedup
with usage-fingerprint fallback, text extraction from ContentBlock arrays,
40×50ms retry loop for Stop-before-JSONL races (PO-5), and last_assistant_message
fallback. All 18 tests in tests/transcript.rs pass; AS-6 verified with
MOCK_DELAY_JSONL=100.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
