[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/fdfd0.py

@@ -67,7 +67,7 @@ def test0(solver=generic_solver):
         num_points = 24,
         )
 
-    dxes = [grid.dxyz, grid.autoshifted_dxyz()]
+    dxes = [grid.autoshifted_dxyz(), grid.dxyz]
     for a in (0, 1, 2):
         for p in (-1, 1):
             dxes = meanas.fdfd.scpml.stretch_with_scpml(dxes, axis=a, polarity=p, omega=omega,