nx_journal_log_test.nx

nx_journal_log_test.nx -- smoke for nx_journal_log ST-3 MVP. Exercises: 1. Allocation + canary + initial state 2. Append + read roundtrip on 3 distinct payloads 3. Count increments correctly 4. read returns correct schema_id per entry 5. read with oversized buffer; read out-of-range seq_no -> -1 6. Empty log -> verify_chain returns -1 (trivially intact); latest_hash returns NX_JOURNAL_NOT_FOUND 7. Filled log -> verify_chain returns -1 (chain intact) 8. latest_hash deterministic across two journals with same payload sequence (cross-journal hash determinism) 9. TAMPER detection: stomp entry K's blob_w0 directly -> verify_chain returns K+1 (next entry's prev_hash no longer matches the recomputed hash of the tampered entry) 10. Canary tamper on log struct -> all ops refuse 11. Bad-input gates (null log / null payload / negative len / out-of-range seq_no)