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/skin.py

@@ -59,7 +59,9 @@ def resistance_factor(thickness_m: float, freq_hz: float,
 
 
 def parse_frequency(text: str) -> float:
-    """'0', '100k', '1.5M', '142500' -> Hz. Empty/invalid -> 0 (DC)."""
+    """'0', '100k', '1.5M', '142500' -> Hz; empty -> 0 (DC).
+    Raises ValueError on unparseable or negative input (a typo silently
+    becoming DC would mislabel the result)."""
     t = text.strip().lower().replace(",", ".").removesuffix("hz").strip()
     if not t:
         return 0.0
@@ -72,7 +74,7 @@ def parse_frequency(text: str) -> float:
         mult, t = 1e3, t[:-1]
     elif t.endswith("g"):
         mult, t = 1e9, t[:-1]
-    try:
-        return max(0.0, float(t) * mult)
-    except ValueError:
-        return 0.0
+    value = float(t) * mult          # ValueError on garbage
+    if value < 0:
+        raise ValueError(f"negative frequency: {text!r}")
+    return value