style: resolve the remaining ruff findings

Auto-fixes (UP037, UP038, F401, I001, SIM105, RUF100) plus hand-fixes:
drop the now-unused `Union` imports, annotate mutable class attributes
with ClassVar, raise ValidationError instead of blind Exception in the
zero-length test, delete two dead locals, and bind the per-iteration
grid-builder closures to their loop variables (B023).

tests/unit/test_grid_system.py

@@ -2,7 +2,6 @@
 
 from __future__ import annotations
 
-import math
 from pathlib import Path
 
 import pytest
@@ -174,7 +173,7 @@ def test_coord_system_round_trip_world_local() -> None:
     p_local = (1.5, -0.5, 2.0)
     p_world = cs.local_to_world(p_local)
     p_back = cs.world_to_local(p_world)
-    for a, b in zip(p_local, p_back):
+    for a, b in zip(p_local, p_back, strict=False):
         assert a == pytest.approx(b, abs=1e-9)