Adaptive grid on by default; finer auto cell size under it

ADAPTIVE_CELLS defaults to True (dialog checkbox stays; untick or
--no-adaptive for the uniform reference grid). With the adaptive grid
the auto cell sizer targets TARGET_CELLS_ADAPTIVE (8M fine cells,
~2x finer h) since unknowns no longer scale with the fine cell count -
memory of the masks/field arrays is the new bound.

The test suite pins ADAPTIVE_CELLS off via an autouse conftest fixture:
the exact-value tests define the uniform reference grid; adaptive tests
opt in per test.

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

fill_resistance/standalone.py

@@ -47,9 +47,11 @@ def main(argv=None) -> int:
     ap.add_argument("--force-iterative", action="store_true",
                     help="use the iterative solver (AMG-CG, or Jacobi-CG "
                          "without pyamg) regardless of problem size")
-    ap.add_argument("--adaptive", action="store_true",
-                    help="solve on the adaptive quadtree grid (coarse "
-                         "plane interiors)")
+    ap.add_argument("--adaptive", action=argparse.BooleanOptionalAction,
+                    default=None,
+                    help="adaptive quadtree grid (coarse plane interiors); "
+                         "default: config (on). --no-adaptive forces the "
+                         "uniform reference grid")
     args = ap.parse_args(argv)
 
     if args.cell_um is not None:
@@ -58,8 +60,8 @@ def main(argv=None) -> int:
         config.INTERACTIVE = False
     if args.force_iterative:
         config.SPSOLVE_MAX_UNKNOWNS = 0
-    if args.adaptive:
-        config.ADAPTIVE_CELLS = True
+    if args.adaptive is not None:
+        config.ADAPTIVE_CELLS = args.adaptive
 
     problem = load_problem(args.dump)
     if args.strip_buildup: