Route to the real board outline, per connection, and out of fine-pitch pins

- The exact engine verified copper per net and dropped a whole net on any
  conflict: a 25-pin +3V3 net lost every connection to one bad segment.
  Verify and accept per ratsnest connection instead.
- Pad and keepout cells were rounded outward, blocking cells up to half a
  step clear of the copper; 62 of the 64 pins of a 0.5 mm pitch LQFP could
  not start a route. Block only cells whose centre is inside the clearance
  zone, and let a net walk along inside its own pad, which adds no copper.
- Routing was bounded by the outline's bounding box, so tracks cut across
  rounded corners. Keep the DSN boundary as a polygon; the grid blocks cells
  too close to it (scanline plus per-edge, precomputed for vias) and the
  exact and room engines reject copper that is.
- --edge-clearance MM sets copper-to-edge clearance, e.g. KiCad's
  min_copper_edge_clearance, which the DSN does not carry. Default 0.

On the TerraHawk rc4 power stage the exact engine now matches the
FreeRouting JAR: same open count, no DRC errors, with a 0.5 mm edge rule.

src/freeroute/board/build.py

@@ -50,6 +50,7 @@ def build_board(dsn: DsnBoard) -> BasicBoard:
     bounding_box = _build_bounding_box(dsn, transform)
 
     board = BasicBoard(layer_structure, nets, clearance, bounding_box)
+    board.outline = _build_outline(dsn, transform)
 
     _insert_pins(dsn, board, transform, terminal_to_net)
     _insert_keepouts(dsn, board, transform)
@@ -99,6 +100,18 @@ def _build_bounding_box(dsn: DsnBoard, transform: CoordinateTransform) -> IntBox
     return IntBox(llx, lly, urx, ury)
 
 
+def _build_outline(dsn: DsnBoard, transform: CoordinateTransform):
+    """The DSN boundary as a polygon, or None when it is not a polygon."""
+    from .outline import BoardOutline
+
+    shape = dsn.boundary
+    if not isinstance(shape, (Polygon, Path)):
+        return None
+    c = shape.coords
+    pts = [transform.dsn_to_board_point(c[i], c[i + 1]) for i in range(0, len(c) - 1, 2)]
+    return BoardOutline(pts) if len(pts) >= 3 else None
+
+
 def _shape_coords(shape: Shape) -> list[float]:
     if isinstance(shape, Rectangle):
         return list(shape.coords)