Model SMD pad copper: exact net pad shapes stamped on their layers

Pads are the junctions where traces and thermal-relief spokes actually
meet; without their copper a multi-track junction necks down to the
accidental overlap of the rounded track ends, or is severed outright.
gather_smd_pad_copper fetches the exact shape of every undrilled pad
on the net (one API call per pad, layer from the padstack) and
build_problem stamps them onto their own layer (INCLUDE_SMD_PADS).
Selected SMD-pad contacts get their real copper as a side effect
(previously electrode-shape intersected whatever lay underneath).
Dead-end pads become floating islands that the existing connectivity
restriction drops.

Closes the documented SMD-pad-copper limitation (prompted by the
padne comparison).

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

fill_resistance/config.py

@@ -31,6 +31,11 @@ CAP_PLATING_UM = 15.0           # cap plating thickness (fab spec)
 CAP_MAX_DRILL_MM = 0.5          # fab caps only small vias: drills above this
                                 # stay open even with VIAS_CAPPED
                                 # (dialog-settable)
+INCLUDE_SMD_PADS = True         # the net's SMD pad copper conducts too (exact
+                                # shapes on the pad's layer): pads are the
+                                # junctions where traces/spokes meet, and
+                                # selected pad contacts get their real copper.
+                                # Dead-end pads are dropped as floating islands
 INCLUDE_TH_PADS = True          # plated through-hole pads stitch layers too;
                                 # their holes are modeled solder-filled (a
                                 # soldered component lead), so the solder core