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/report.py

@@ -102,8 +102,11 @@ def write_summary(outdir: Path, problem: Problem, stack: RasterStack,
         f"solver:            {info.method}"
         + (f", {info.iterations} iters, residual {info.residual:.2e}"
            if info.iterations is not None else ""),
-        f"I1/I2 @ 1V:        {result.I1_a:.9g} / {result.I2_a:.9g} A "
-        f"(mismatch {result.mismatch_rel:.2e})",
+        (f"I1/I2 @ 1V:        {result.I1_a:.9g} / {result.I2_a:.9g} A "
+         f"(mismatch {result.mismatch_rel:.2e})"
+         if result.contact_model == "equipotential" else
+         f"solve residual:    {result.mismatch_rel:.2e} "
+         f"(KCL, prescribed injection)"),
         f"timings [s]:       "
         f"{', '.join(f'{k}={v:.2f}' for k, v in result.timings.items())}",
         "",