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_contacts.py

@@ -3,9 +3,10 @@ import numpy as np
 import pytest
 
 from fill_resistance import raster, solver
-from fill_resistance.errors import ElectrodeError
+from fill_resistance.errors import ConnectivityError, ElectrodeError
 from fill_resistance.geometry import Electrode
-from tests.util import NM, make_problem, rect_mm, sigma_s, strip_problem
+from tests.util import (NM, make_multilayer, make_problem, rect_mm, sigma_s,
+                        strip_problem)
 
 
 def _solve(problem, h_mm, model, i_test=1.0):
@@ -161,6 +162,22 @@ def test_injection_area_partition_first_wins():
     assert total == pytest.approx(1.0, rel=1e-12)
 
 
+def test_uniform_multicomponent_raises():
+    """Two disconnected sheets that each touch both terminals: the
+    uniform model would build a singular system (one ground cell, pure-
+    Neumann second component) and previously returned garbage silently
+    (e.g. negative gigaohms). It must refuse; equipotential handles it."""
+    strip1 = [(0, 0), (10, 0), (10, 1), (0, 1)]
+    strip2 = [(0, 0), (10, 0), (10, 2), (0, 2)]   # asymmetric shares
+    p = make_multilayer([[(strip1, [])], [(strip2, [])]],
+                        (0, 0, 1, 2), (9, 0, 10, 1))  # contact 'all', no vias
+    with pytest.raises(ConnectivityError, match="disconnected"):
+        _solve(p, 1.0, "uniform")
+    res, _ = _solve(p, 1.0, "equipotential")
+    assert np.isfinite(res.R_ohm) and res.R_ohm > 0
+    assert res.power_balance_rel < 1e-9
+
+
 def test_touching_ok_uniform_error_equipotential():
     p = make_problem([([(0, 0), (10, 0), (10, 10), (0, 10)], [])],
                      rect1_mm=(0, 0, 5, 10), rect2_mm=(5, 0, 10, 10))

tests/test_geometry.py

@@ -51,6 +51,7 @@ def test_problem_json_roundtrip_v2(tmp_path):
          [([(0, 0), (10, 0), (10, 1), (0, 1)], [])]],
         rect1_mm=(0, 0, 1, 1), rect2_mm=(9, 0, 10, 1),
         contact1="L0", contact2="L1", vias_mm=[(5.5, 0.5)])
+    p.vias[0].pad_nm = 600_000
     f = tmp_path / "dump.json"
     save_problem(p, f)
     q = load_problem(f)
@@ -58,6 +59,7 @@ def test_problem_json_roundtrip_v2(tmp_path):
     assert q.electrodes1[0].contact == "L0"
     assert q.electrodes2[0].contact == "L1"
     assert len(q.vias) == 1 and q.vias[0].drill_nm == p.vias[0].drill_nm
+    assert q.vias[0].pad_nm == 600_000
     assert q.plating_nm == p.plating_nm
     assert np.array_equal(q.layers[0].polygons[0].outline,
                           p.layers[0].polygons[0].outline)

tests/test_multilayer.py

@@ -74,9 +74,10 @@ def test_parallel_vias_halve_barrel_resistance():
 
 
 def test_antipad_bridging():
-    """3 layers; the middle layer has an antipad hole at the via cell, so
-    the barrel bridges L0 -> L2 directly with DOUBLE the length."""
-    mid_with_hole = [(STRIP, [[(5, 0), (6, 0), (6, 1), (5, 1)]])]
+    """3 layers; the middle layer has an antipad hole at the via, WIDER
+    than the barrel connection search (pad footprint + 1 cell), so the
+    barrel bridges L0 -> L2 directly with DOUBLE the length."""
+    mid_with_hole = [(STRIP, [[(4, 0), (7, 0), (7, 1), (4, 1)]])]
     p = make_multilayer(
         [[(STRIP, [])], mid_with_hole, [(STRIP, [])]],
         rect1_mm=(0, 0, 1, 1), rect2_mm=(9, 0, 10, 1),
@@ -88,6 +89,43 @@ def test_antipad_bridging():
     assert res.R_ohm == pytest.approx(r_exact, rel=1e-9)
 
 
+def test_thermal_gap_via_connects_to_nearby_copper():
+    """The cell under the via is not copper (thermal-relief knockout),
+    but fill copper within the pad footprint (+1 cell) still reaches the
+    barrel: the link lands on the nearest copper cell instead of being
+    silently dropped."""
+    top_with_gap = [(STRIP, [[(5, 0), (7, 0), (7, 1), (5, 1)]])]
+    p = make_multilayer(
+        [top_with_gap, [(STRIP, [])]],
+        rect1_mm=(0, 0, 1, 1), rect2_mm=(9, 0, 10, 1),
+        contact1="L0", contact2="L1",
+        vias_mm=[(5.5, 0.5)], gap_mm=1.0)
+    res, _ = _solve(p, 1.0)
+    sig = sigma_s()
+    # L0 attaches at col 4 (nearest copper, 1.0 mm from the barrel),
+    # L1 at col 5: 4 faces on L0, the barrel, 4 faces on L1 - exact
+    r_exact = (4 + 4) / sig + _r_via(1.0)
+    assert res.R_ohm == pytest.approx(r_exact, rel=1e-9)
+    assert len(res.via_reports) == 1
+    assert res.via_reports[0].current_a == pytest.approx(1.0, rel=1e-9)
+
+
+def test_dead_barrel_is_warned(capsys):
+    """A via isolated from the fill by an antipad wider than the search
+    radius on all but one layer carries nothing and is reported."""
+    bot_with_hole = [(STRIP, [[(3, 0), (8, 0), (8, 1), (3, 1)]])]
+    p = make_multilayer(
+        [[(STRIP, [])], bot_with_hole],
+        rect1_mm=(0, 0, 1, 1), rect2_mm=(9, 0, 10, 1),
+        contact1="L0", contact2="L0",
+        vias_mm=[(5.5, 0.5)], gap_mm=1.0)
+    res, _ = _solve(p, 1.0)
+    sig = sigma_s()
+    assert res.R_ohm == pytest.approx(9 / sig, rel=1e-9)   # L0 alone
+    assert res.via_reports == []
+    assert "carry no current" in capsys.readouterr().out
+
+
 def test_via_short_between_electrodes_raises():
     """Both electrodes over the SAME cell on different layers with a via
     there = direct short, no free copper -> error."""

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)

tests/test_skin.py

@@ -58,7 +58,10 @@ def test_parse_frequency():
     assert skin.parse_frequency("2meg") == 2_000_000.0
     assert skin.parse_frequency("100000") == 100_000.0
     assert skin.parse_frequency("100 kHz") == 100_000.0
-    assert skin.parse_frequency("junk") == 0.0
+    with pytest.raises(ValueError):
+        skin.parse_frequency("junk")      # must not silently become DC
+    with pytest.raises(ValueError):
+        skin.parse_frequency("-5k")
 
 
 def test_single_layer_ac_scales_exactly():

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(