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)

tests/test_quadtree.py

@@ -56,13 +56,13 @@ def test_uniform_limit_graph_identical():
     ny, nx = stack.shape2d
     flat_of_leaf = grid.y0.astype(np.int64) * nx + grid.x0
     ia, ib, g = quadtree.leaf_edges(grid, p.sigma_s(0))
-    ours = np.sort(np.stack([
+    ours = np.stack([
         np.minimum(flat_of_leaf[ia], flat_of_leaf[ib]),
-        np.maximum(flat_of_leaf[ia], flat_of_leaf[ib])], axis=1), axis=0)
+        np.maximum(flat_of_leaf[ia], flat_of_leaf[ib])], axis=1)
 
     edges = solver.build_edges(stack, p, [p.sigma_s(0)])
-    ref = np.sort(np.stack([np.minimum(edges.a, edges.b),
-                            np.maximum(edges.a, edges.b)], axis=1), axis=0)
+    ref = np.stack([np.minimum(edges.a, edges.b),
+                    np.maximum(edges.a, edges.b)], axis=1)
     assert ours.shape == ref.shape
     assert np.array_equal(np.sort(ours.view("i8,i8"), order=["f0", "f1"],
                                   axis=0),