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>

tests/test_raster.py

@@ -90,6 +90,14 @@ def test_hard_max_cells_guard(monkeypatch):
         raster.choose_cell_size(p.copper_bbox(), len(p.layers))
 
 
+def test_cell_override_nonpositive_raises(monkeypatch):
+    p = strip_problem()
+    for bad in (0.0, -50.0):
+        monkeypatch.setattr(config, "CELL_UM_OVERRIDE", bad)
+        with pytest.raises(GridSizeError, match="positive"):
+            raster.choose_cell_size(p.copper_bbox(), len(p.layers))
+
+
 def test_auto_cell_size_hits_target():
     # large plane: unclamped regime, cell count tracks TARGET_CELLS
     p = strip_problem(length=200, width=100, e_len=5)