feat(m1): offline generation pipeline + CLI (T1.5)

- tsmm.data.gen_pipeline.gen_one_sample: composes attrs/series/anomaly/event/missing/instruct
  into one JSON-serializable sample (full schema)
- write_jsonl helper
- scripts/gen_synthetic.py: multiprocessing + tqdm CLI (--n/--out/--seed/--workers)
- regression: build_instruction('event') with empty events no longer raises (TDD RED first)
- tests/test_gen_pipeline.py (6 tests, RED->GREEN)

Exit criteria: python scripts/gen_synthetic.py --n 1000 -> 1000 samples, schema-OK,
all 6 categories balanced (~130-190 each).

tests/test_instruct.py

@@ -97,6 +97,15 @@ def test_event_answer_references_the_event():
     assert "deploy" in (instruction + " " + answer).lower()
 
 
+def test_event_category_handles_missing_events_gracefully():
+    """Regression: selecting the 'event' category when events==[] must not raise."""
+    ctx = _sample_context()
+    ctx["events"] = []
+    instruction, answer = build_instruction(category="event", **ctx, seed=0)
+    assert isinstance(instruction, str) and instruction.strip()
+    assert isinstance(answer, str) and answer.strip()
+
+
 def test_root_cause_answer_is_text():
     ctx = _sample_context()
     _, answer = build_instruction(category="root_cause", **ctx, seed=0)