Fix swarm-review findings: empty-layer crashes, teardrop fills, Jmag

- adaptive: skip layers with zero quadtree leaves in the connectivity
  restriction and mesh-boundary loops (IndexError on boards where a
  selected layer has no copper)
- board_io: accept ZT_TEARDROP zones as conducting copper (KiCad types
  teardrop fills ZT_TEARDROP, never ZT_COPPER, so they were dropped)
- geometry: copper_bbox uses the exact stroke bbox (centerline extrema
  + half width) instead of a 100 um chord tessellation that could
  undershoot arc/cap extrema past the raster guard margin
- solver/adaptive: reference |J| to the conduction-equivalent thickness
  sigma*rho in every branch (the uniform branch used geometric t, so AC
  plots changed scale ~rs_ratio depending on unrelated per-cell maps)
- solver/adaptive/raster: chain cells no longer show phantom sheet-face
  currents; store dl per chain link and overlay the true 1D density
  |dV|/(rho*dl) (exact at any frequency: AC scaling of link conductance
  and cross-section cancels)
- test_quadtree: compare edge lists pair-for-pair (the independent
  column sort destroyed endpoint association)

fill_resistance/board_io.py

@@ -292,7 +292,9 @@ def gather_net_fills(board: Board) -> dict[str, dict[str, list[Polygon]]]:
     """net -> layer_name -> merged fill polygons (non-empty only)."""
     fills: dict[str, dict[str, list[Polygon]]] = {}
     for zone in board.get_zones():
-        if zone.type != ZoneType.ZT_COPPER:
+        # teardrop fills are conducting copper too, but KiCad types them
+        # ZT_TEARDROP instead of ZT_COPPER
+        if zone.type not in (ZoneType.ZT_COPPER, ZoneType.ZT_TEARDROP):
             continue
         net = zone.net.name if zone.net is not None else "<no net>"
         for layer, polys in zone.filled_polygons.items():
@@ -392,8 +394,8 @@ def gather_mask_buildups(board: Board) -> dict[str, list[Polygon]]:
 
 
 def any_zone_unfilled(board: Board) -> bool:
-    return any(z.type == ZoneType.ZT_COPPER and not z.filled
-               for z in board.get_zones())
+    return any(z.type in (ZoneType.ZT_COPPER, ZoneType.ZT_TEARDROP)
+               and not z.filled for z in board.get_zones())
 
 
 def refill(board: Board) -> None: