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>

plugin.json

@@ -2,7 +2,7 @@
     "$schema": "https://go.kicad.org/api/schemas/v1",
     "identifier": "th.co.b4l.fill-resistance",
     "name": "Fill Resistance",
-    "description": "DC resistance of a copper zone fill between two rectangle electrodes",
+    "description": "DC/AC resistance of copper zone fills between two contacts (marker rectangles or pads), single- or multi-layer with via coupling",
     "runtime": {
         "type": "python"
     },
@@ -10,7 +10,7 @@
         {
             "identifier": "fill-resistance-run",
             "name": "Fill Resistance",
-            "description": "Select two rectangles marking the contact areas, then run to compute the fill resistance between them",
+            "description": "Mark the contacts (rectangles on the User.1/User.2 marker layers and/or selected pads), then run to compute the fill resistance between them",
             "entrypoint": "fill_res_action.py",
             "show-button": true,
             "scopes": ["pcb"],