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>

README.md

@@ -70,18 +70,21 @@ SWIG API. Requires KiCad **10.0.1+**.
   barrel lengths.
 - Via/pad barrels: thin-wall annulus, R = ρ·L/(π·d·t_plating),
   `VIA_PLATING_UM = 18` in `fill_resistance/config.py`. Vias are always
-  plated. **Via capping is not modeled.** Layer-to-layer it cannot
-  matter at DC: the cap (≥15 µm plating per fab spec, thinner than the
-  foil) sits over the hole mouth in parallel with the annular-ring
-  contact, not in series. In-plane, the model treats every via mouth as
-  solid layer-thickness copper (KiCad fills do not subtract the drill),
-  where reality is a hole (uncapped) or the thin cap. The error is
-  bounded by the dilute-hole correction ρ_eff ≈ ρ·(1 + 2f) inside via
-  fields (f = mouth area fraction; 0.3 mm drills on a 1 mm grid give
-  f ≈ 7 % → ≈ +14 % locally), is partially offset by the unmodeled
-  annular-ring copper, and largely vanishes where the current descends
-  into the barrels anyway — typically ≪ a few % of the total R.
-  Per layer a barrel attaches to
+  plated. Each via also contributes its **ring/pad copper** (a
+  full-thickness disc of the pad diameter on every spanned layer) and
+  its **drill mouth**, area-weighted per cell: with the **"vias filled +
+  capped" checkbox** (default on, `VIAS_CAPPED`) the mouth carries a
+  thin copper cap (`CAP_PLATING_UM = 15`, fab spec) on the **outer**
+  layers and is an open hole on inner layers; unchecked, mouths are open
+  holes everywhere. Layer-to-layer the cap never matters at DC (it is in
+  parallel with the annular-ring contact, not in series) — the checkbox
+  only affects in-plane conduction across outer-layer mouths. Sub-cell
+  mouths scale their cells' sheet conductance by the true covered
+  fraction (4×4 supersampling), so coarse grids see the correct small
+  perturbation instead of a whole-cell hole. THT-pad copper and drills
+  remain outside the model; at f > 0 the thickness scaling is applied
+  multiplicatively to the skin-corrected sheet conductance
+  (approximation). Per layer a barrel attaches to
   the fill cell under it, or to the nearest copper cell within the pad
   footprint plus one grid cell — fills joined by **thermal-relief
   spokes** still connect; wider antipads do not, and the barrel bridges