Model pads and vias as KiCad places them

Measured against KiCad's own pads on the TerraHawk rc4 power stage:

- Back-side parts kept their pads on F.Cu (32 pads). Mirror the copper
  layer for a back-side part.
- Pad outlines were copied unrotated (29 pads on rotated parts). Place the
  outline through the same chain as the pin: pin rotation, back-side
  mirror, part rotation. Oval pads, a path with a width, came out as a
  zero-height sliver; pad them out by the aperture.
- Multi-shape padstacks used the first shape; use the largest.
- Footprint-level keepouts were parsed but never placed. KiCad exports a
  mounting hole's plated ring that way, so the router could not see it.
- Vias were sized like a trace (0.25 mm square for a 0.6 mm via), and every
  remaining short and clearance error was a via. Read the size from the
  DSN's via padstack and use it in all three engines; the grid engine now
  reserves the cells a via's copper reaches.

With these the exact and room engines route that board with no shorts,
clearance or other DRC errors. Pins now carry their component reference.

tests/board/test_build.py

@@ -63,11 +63,20 @@ def test_pin_count_matches_placement(dsn, board):
 
 
 def test_obstacle_count_matches_keepouts(dsn, board):
-    # each keepout expands to one obstacle per applicable layer; with no keepouts
-    # in this fixture the count is zero
-    assert len(board.get_obstacle_areas()) >= len(dsn.keepouts)
+    # Board-level keepouts plus every placed instance of a footprint-level one.
+    # This fixture has none of the first and six plated mounting holes, whose
+    # copper KiCad exports as a keepout on each layer.
+    images = {img.name: img for img in dsn.images}
+    placed = 0
+    for placement in dsn.placements:
+        image = images.get(placement.lib_name)
+        if image is None:
+            continue
+        per_part = len(image.keepouts) + len(image.via_keepouts) + len(image.wire_keepouts)
+        placed += per_part * sum(p.x is not None for p in placement.places)
     assert len(dsn.keepouts) == 0
-    assert board.get_obstacle_areas() == []
+    assert placed == 12
+    assert len(board.get_obstacle_areas()) == placed
 
 
 # --- per-item invariants ----------------------------------------------------