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/pipeline.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 from pathlib import Path
 
 from . import config, plots, raster, report, solver
+from .errors import UserFacingError
 from .geometry import Problem
 from .solver import Result
 
@@ -14,6 +15,8 @@ def run(problem: Problem, outdir: Path | None, show: bool = True,
         contact_model: str | None = None) -> Result:
     if i_test is None:
         i_test = config.TEST_CURRENT_A
+    if i_test <= 0:
+        raise UserFacingError(f"Test current must be > 0 A (got {i_test:g}).")
     h = raster.choose_cell_size(problem.copper_bbox(), len(problem.layers))
     print(f"rasterizing {len(problem.layers)} layer(s) at cell size "
           f"{h / 1000:.1f} um ...")