Model via ring copper and drill mouths (capping), dialog-toggleable

Each via now contributes its ring/pad copper (full-thickness disc of
the pad diameter on every spanned layer) and its drill mouth: with
"vias filled + capped" (dialog checkbox, default on, VIAS_CAPPED) the
mouth carries a CAP_PLATING_UM (15 um) thin copper cap on the outer
layers and is an open hole on inner layers; unchecked, mouths are open
everywhere. Mouth coverage is area-weighted per cell (4x4
supersampling) through a per-cell thickness map feeding the existing
harmonic-mean face machinery, so sub-cell mouths perturb the sheet by
their true covered fraction instead of whole cells. Fully swallowed
cells leave the mask; the barrel then attaches through the ring via the
existing pad-footprint search. THT-pad copper and drills stay outside
the model. Ring discs paint before 1D trace chains (chains see them as
regular copper), mouths after wide tracks (drills go through trace
copper). standalone gains --uncapped.

Tests: cap==foil identity against the feature-off reference, strict
R(solid) < R(cap) < R(hole) ordering, ring bridging a fill gap that a
ringless barrel cannot cross, gentle sub-cell perturbation at coarse
grids, and JSON roundtrip of the new fields.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

fill_resistance/dialog.py

@@ -36,6 +36,7 @@ class Selection:
     include_buildup: bool = False
     extra_cu_um: float = 0.0
     include_tracks: bool = True
+    vias_capped: bool = True
 
 
 class _Dialog(QDialog):
@@ -65,6 +66,12 @@ class _Dialog(QDialog):
         self.tracks_check.setChecked(config.INCLUDE_TRACKS)
         form.addRow("Conductors:", self.tracks_check)
 
+        self.capped_check = QCheckBox(
+            f"vias filled + capped ({config.CAP_PLATING_UM:g} µm cap; "
+            f"off = open mouths)")
+        self.capped_check.setChecked(config.VIAS_CAPPED)
+        form.addRow("Vias:", self.capped_check)
+
         self.contact1_box = QComboBox()
         self.contact2_box = QComboBox()
         form.addRow(f"V+ ({e1_label}):", self.contact1_box)
@@ -206,7 +213,8 @@ class _Dialog(QDialog):
                          contact_model=self.model_box.currentData(),
                          include_buildup=self.buildup_check.isChecked(),
                          extra_cu_um=extra_cu,
-                         include_tracks=self.tracks_check.isChecked())
+                         include_tracks=self.tracks_check.isChecked(),
+                         vias_capped=self.capped_check.isChecked())
 
     def _try_accept(self) -> None:
         try: