Fix solver correctness and input-validation issues from code review

- Refuse the uniform contact model when the fills form multiple
  disconnected copper groups that each touch both terminals: the
  prescribed injection split is ill-posed and the grounded system was
  singular, silently returning garbage (e.g. negative gigaohms).
  connected_restrict now reports the component count; a power-balance
  backstop (SolverError) catches any other inconsistent solve.
- Connect via/pad barrels to the nearest fill copper within the pad
  footprint (+1 cell) instead of only the exact center cell, so
  thermal-relief spokes still stitch layers; barrels that reach fill on
  fewer than two layers are warned about. ViaLink gains pad_nm
  (extracted from the padstack, JSON-roundtripped).
- Validate dialog input on OK (layers, current > 0, cell > 0, parseable
  frequency, extra Cu >= 0) with an inline error instead of silently
  substituting defaults; parse_frequency raises on garbage; pipeline
  rejects i_test <= 0; choose_cell_size rejects non-positive overrides.
- Warn when a contact part is dropped by the connectivity restriction;
  floor instead of truncate in cell_of; correct the uniform-model
  summary line; drop an unused variable; refresh plugin.json wording.

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

fill_resistance/standalone.py

@@ -16,6 +16,7 @@ from pathlib import Path
 from . import config, pipeline
 from .errors import UserFacingError
 from .geometry import load_problem
+from .skin import parse_frequency
 
 
 def main(argv=None) -> int:
@@ -23,7 +24,7 @@ def main(argv=None) -> int:
     ap.add_argument("dump", type=Path, help="geometry_dump.json from a plugin run")
     ap.add_argument("--current", type=float, default=None,
                     help="test current [A] (default: config TEST_CURRENT_A)")
-    ap.add_argument("--freq", type=str, default="0",
+    ap.add_argument("--freq", type=parse_frequency, default=0.0,
                     help="frequency, e.g. 142k or 1.5M (default: DC). "
                          "AC results are a lower bound (skin per foil only)")
     ap.add_argument("--cell-um", type=float, default=None,
@@ -64,11 +65,10 @@ def main(argv=None) -> int:
                   file=sys.stderr)
             return 1
 
-    from .skin import parse_frequency
     outdir = args.out if args.out is not None else args.dump.parent
     try:
         pipeline.run(problem, outdir, show=not args.no_show,
-                     i_test=args.current, freq_hz=parse_frequency(args.freq),
+                     i_test=args.current, freq_hz=args.freq,
                      contact_model=args.contact_model)
     except UserFacingError as e:
         print(f"ERROR: {e}", file=sys.stderr)