Give pin stubs clearance, and let a route leave a pin by any open side

Every route starts and ends with a stub from the pin to its first grid
cell, and the grid never saw that copper: on the rc4 sense board 24 of 39
connections the exact verifier rejected had a later route pass beside an
earlier stub. A route's stubs now claim the cells they come near, a stub
must be clear of other nets to be used, and a route may start from any
enterable neighbour of the pin cell, costed by stub length (Manhattan on
the orthogonal track, so a pin on a cell centre still leaves straight).

Tests: a route near a rounded corner keeps its edge clearance; and a
strict xfail that pins down the remaining gap, a 0.5 mm pin row the grid
cannot give a column to every pin of.

tests/route/test_real_board_geometry.py

@@ -0,0 +1,66 @@
+"""Routing behaviours found on the TerraHawk rc4 boards (2026-09-27)."""
+
+from __future__ import annotations
+
+import math
+
+import pytest
+
+from freeroute.board import build_board
+from freeroute.dsn import parse_dsn
+from freeroute.route.pipeline import route_dsn_board_exact
+
+# 20 x 20 mm board whose corners are rounded with a 3 mm radius, as polylines
+_R = 3000.0
+_arc = [(_R - _R * math.cos(math.radians(a)), _R - _R * math.sin(math.radians(a))) for a in range(0, 91, 15)]
+
+
+def _outline_coords():
+    pts = []
+    for cx, cy, sx, sy in ((0, 0, 1, 1), (20000, 0, -1, 1), (20000, -20000, -1, -1), (0, -20000, 1, -1)):
+        corner = [(cx + sx * x, cy - sy * y) for x, y in _arc]
+        pts.extend(corner if sx * sy > 0 else corner[::-1])
+    return " ".join(f"{x:.1f} {y:.1f}" for x, y in pts)
+
+
+def _dsn(placement, network, extra=""):
+    return f"""(pcb t (parser) (resolution um 10) (unit um)
+  (structure (layer F.Cu (type signal)) (layer B.Cu (type signal))
+    (boundary (path pcb 0 {_outline_coords()}))
+    (via "Via[0-1]_600:300_um") (rule (width 250) (clearance 150)) {extra})
+  (placement {placement})
+  (library (image P (pin pad 1 0 0))
+    (padstack pad (shape (rect F.Cu -150 -500 150 500)) (attach off))
+    (padstack "Via[0-1]_600:300_um" (shape (circle F.Cu 600)) (shape (circle B.Cu 600)) (attach off)))
+  (network {network}))"""
+
+
+def test_routes_keep_edge_clearance_at_a_rounded_corner():
+    # two pads near the top-left corner: the direct path cuts the corner arc
+    text = _dsn("(component P (place A 1800 -6000 front 0) (place B 6000 -1800 front 90))",
+                "(net N (pins A-1 B-1))")
+    dsn = parse_dsn(text)
+    result, scale, _ = route_dsn_board_exact(dsn, edge_clearance_mm=0.5)
+    assert result.routed_net_numbers
+    outline = build_board(dsn).outline
+    half = 125 * scale
+    for segs in result.result.wires.values():
+        for _layer, pts in segs:
+            for p in pts:
+                # every trace point, plus half the trace, keeps 0.5 mm to the edge
+                assert outline.edge_distance(p) >= half + 500 * scale - 1
+
+
+@pytest.mark.xfail(
+    strict=True,
+    reason="known gap: the grid step (trace + clearance, 0.4 mm here) does not line up "
+    "with a 0.5 mm pin row, so only the outer pins get a column; the FreeRouting JAR, "
+    "being gridless, routes all five. This is most of the gap on an LQFP.",
+)
+def test_every_pin_of_a_half_millimetre_pitch_row_escapes():
+    # five pads at 0.5 mm pitch, each wired to its own pad 8 mm away
+    places = " ".join(f"(place R{i} {6000 + 500 * i} -6000 front 0)" for i in range(5))
+    places += " " + " ".join(f"(place F{i} {6000 + 500 * i} -14000 front 0)" for i in range(5))
+    nets = " ".join(f"(net N{i} (pins R{i}-1 F{i}-1))" for i in range(5))
+    result, _, _ = route_dsn_board_exact(parse_dsn(_dsn(f"(component P {places})", nets)))
+    assert len(result.routed_net_numbers) == 5