Route 45-degree traces on the exact track (opt-in diagonal mode)

Add diagonal=True to the exact router: after orthogonal routing, each 2-pin net
is retried as the shortest exactly-clean octilinear (0/45/90/135-degree) trace.
A dropped net is recovered; an orthogonal route is replaced only when the
diagonal is strictly shorter. Candidates are the direct 45-degree segment or the
two diagonal-plus-axis two-benders; each segment's diagonal copper is covered by
the exact octagon and its clearance checked with clearance_conflict_shape, so a
trace fits a diagonal corridor the bounding-box cover would reject.

Endpoints stay on the pads; diagonal copper is stored with its exact octagon so
later nets clear it precisely; same-net copper is ignored during the check so the
candidate is verified against every other net before the orthogonal copper is
ripped. With diagonal=False the pass is skipped and output is byte-for-byte the
orthogonal router. Threaded through route_dsn_board_exact /
build_exact_routing_result / route(engine="exact").

src/freeroute/route/pipeline.py

@@ -111,11 +111,13 @@ def route_dsn_board_exact(
     max_passes: int = 10,
     rip_up: bool = True,
     shove: bool = False,
+    diagonal: bool = False,
 ) -> tuple[ExactRouteResult, int, list[str]]:
     """Route a parsed :class:`DsnBoard` with the exact-geometry (orthogonal,
     DRC-verified) track; return the exact result, the scale, and layer names.
     ``shove`` enables moving existing traces aside to recover dropped nets;
-    ``rip_up`` toggles the underlying grid's rip-up-and-retry."""
+    ``rip_up`` toggles the underlying grid's rip-up-and-retry; ``diagonal`` enables
+    the 45-degree recovery pass for still-dropped 2-pin nets."""
     board = build_board(dsn)
     scale = max(dsn.resolution.value, 1)
     width_board = round(_rule_width_dsn(dsn) * scale)
@@ -128,15 +130,18 @@ def route_dsn_board_exact(
         max_passes=max_passes,
         rip_up=rip_up,
         shove=shove,
+        diagonal=diagonal,
     )
     return exact, scale, [layer.name for layer in dsn.layers]
 
 
 def build_exact_routing_result(
-    dsn: DsnBoard, *, layers: list[int] | None = None, shove: bool = False
+    dsn: DsnBoard, *, layers: list[int] | None = None, shove: bool = False, diagonal: bool = False
 ) -> RoutingResult:
     """Route ``dsn`` (exact track) and convert to a DSN-unit RoutingResult."""
-    exact, scale, layer_names = route_dsn_board_exact(dsn, layers=layers, shove=shove)
+    exact, scale, layer_names = route_dsn_board_exact(
+        dsn, layers=layers, shove=shove, diagonal=diagonal
+    )
     return _to_routing_result(exact.result, dsn, scale, layer_names)
 
 
@@ -177,6 +182,7 @@ def route(
     engine: str = "grid",
     shove: bool = False,
     pack: bool = False,
+    diagonal: bool = False,
 ) -> str:
     """Route a Specctra DSN string and return the routed SES string.
 
@@ -190,7 +196,7 @@ def route(
     """
     dsn = parse_dsn(dsn_text)
     if engine == "exact":
-        result = build_exact_routing_result(dsn, layers=layers, shove=shove)
+        result = build_exact_routing_result(dsn, layers=layers, shove=shove, diagonal=diagonal)
     elif engine == "room":
         result = build_rooms_routing_result(dsn, layers=layers, shove=shove, pack=pack)
     else: