test(m4): instruct_check unit tests + unbuffered training scripts

- tests/test_instruct_check.py: cover load_by_category (bucketing/cap/seed
  determinism/short-pool/blank-line) and _forecast_list (nested/flat/
  dict/garbage). 8 tests pass.
- train_stage{1,2}.sh: export PYTHONPATH=src + PYTHONUNBUFFERED=1 so future
  background runs stream logs live (current stage2 run is block-buffered).

scripts/train_stage2.sh

@@ -8,6 +8,7 @@ STAGE1_CKPT="${3:-checkpoints/stage1/stage1_final.pt}"
 CKPT_DIR="${4:-checkpoints/stage2}"
 
 cd "$(dirname "$0")/.."
+export PYTHONPATH=src PYTHONUNBUFFERED=1
 .venv/bin/python -m tsmm.train.stage2 \
   --data "$DATA" \
   --stage1_ckpt "$STAGE1_CKPT" \