[fdmath] correct Gridlock Yee spacing order

Use center-to-center intervals for forward E derivatives and base-cell widths for backward H derivatives in the Gridlock examples. Document the distinction from meshes defined directly by forward intervals and fix the random fixture's backward averaging direction.

Add coordinate-derivative regression coverage for graded periodic and nonperiodic Gridlock meshes using both functional and sparse operators.

Validation: 1689 tests passed, 586 skipped; no new Ruff findings in changed Python files.

examples/eme.py

@@ -109,7 +109,7 @@ def build_geometry(
         z=Extent(min=0, max=1e6),
     )
 
-    dxes = [grid.dxyz, grid.autoshifted_dxyz()]
+    dxes = [grid.autoshifted_dxyz(), grid.dxyz]
     dxes_2d = [[d[0], d[1]] for d in dxes]
     return grid, epsilon, dxes_2d, omega