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_solver.py

@@ -112,6 +112,17 @@ def test_test_current_scaling():
     assert r10.P_total == pytest.approx(100 * r1.P_total, rel=1e-9)
 
 
+def test_nonpositive_test_current_rejected():
+    """i_test <= 0 would divide by zero in the percentage reporting;
+    it must be rejected up front with a clean user-facing message."""
+    from fill_resistance import pipeline
+    from fill_resistance.errors import UserFacingError
+    p = strip_problem()
+    for bad in (0.0, -1.0):
+        with pytest.raises(UserFacingError, match="Test current"):
+            pipeline.run(p, None, show=False, i_test=bad)
+
+
 def test_power_identity():
     """Sum of edge powers equals I^2 R exactly for the direct solve."""
     p = make_problem(