Add data-integrity guards against silent NaN/Inf propagation and races

- log_transform / _validate_unit_pre_dir now raise on non-finite input
  instead of letting a NaN row silently poison the persisted normalizer
  cache (norm < 1e-6 was always False for NaN, so the existing guard
  never caught it).
- encode_secondaries warns when a row's secondary energies cumulatively
  exceed e_sec, instead of silently saturating the overflowing slot's
  stick-breaking logit via the _EPS floor.
- EVENT_ID_FILE_STRIDE overflow now raises instead of silently colliding
  two files' event ids together (reintroducing train/val leakage).
- make_event_split(val_fraction=0.0) now actually holds out nothing,
  instead of always forcing at least 1 validation event.
- setup_cache.save() is now serialized with a flock, since two
  concurrent writers (a real scenario on this repo's shared
  portal/condor machines) could otherwise race and silently drop one
  writer's freshly-computed cache section.
- Documented (no behavior change) the pre_dir ≈ -ẑ antipodal rotation
  singularity in _rodrigues_axis, which is real but inherent to any
  single-valued local-frame convention.

Each fix has a regression test.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_loader.py

@@ -344,6 +344,15 @@ def test_load_event_ids_applies_offset(tmp_path):
     )
 
 
+def test_load_event_ids_raises_when_event_id_reaches_stride(tmp_path):
+    """A raw event_id >= EVENT_ID_FILE_STRIDE would collide into the next
+    file's offset block if silently allowed through — must raise instead."""
+    path = tmp_path / "a.parquet"
+    pd.DataFrame({"event_id": [0, 1, EVENT_ID_FILE_STRIDE]}).to_parquet(path)
+    with pytest.raises(ValueError, match="EVENT_ID_FILE_STRIDE"):
+        load_event_ids(path)
+
+
 def test_load_steps_applies_offset_to_event_id(tmp_path):
     path = tmp_path / "a.parquet"
     _steps_df([0, 1]).to_parquet(path)