--window-reroute N: route a finer grid around each connection still stuck

After the exact engine, every net routed wholly inside a window around a
stuck connection's pins is ripped, the rest of the board is laid down as
fixed copper, and the window alone is routed again at --window-resolution
(default 4) with the stuck net and the ripped nets only. The result is kept
only if fewer connections are left unrouted; each round doubles the margin.
The grid and exact routers gain only_nets and bounds for it. Off by default.

tests/route/test_window_reroute.py

@@ -0,0 +1,95 @@
+"""Window re-route: a finer grid around a stuck connection, kept only when
+it leaves fewer connections unrouted."""
+
+from __future__ import annotations
+
+from freeroute.board import build_board
+from freeroute.dsn import parse_dsn
+from freeroute.geometry import IntBox
+from freeroute.route.grid_router import GridRouter
+from freeroute.route.pipeline import _rule_clearance_dsn, _rule_width_dsn, route_dsn_board_exact
+
+
+def _slot(gap: int = 640, far: bool = False) -> str:
+    """A wall across the board with a slot in it. A trace fits the slot
+    only along a narrow band of heights that the board's 1x grid has no row
+    in and a 4x grid does. ``far`` adds a second net nowhere near the wall."""
+    yc = -5000
+    lo, hi = yc - gap // 2, yc + gap // 2
+    extra_place = "(place Q1 2000 -1000 front 0) (place Q2 6000 -1000 front 0)" if far else ""
+    extra_net = "(net M (pins Q1-1 Q2-1))" if far else ""
+    return f"""(pcb "slot.dsn"
+  (parser (string_quote ") (space_in_quoted_tokens on) (host_cad "t") (host_version "1"))
+  (resolution um 10)
+  (unit um)
+  (structure
+    (layer F.Cu (type signal) (property (index 0)))
+    (boundary (path pcb 0  0 0  20000 0  20000 -10000  0 -10000  0 0))
+    (keepout "" (rect F.Cu 9000 -10000 11000 {lo}))
+    (keepout "" (rect F.Cu 9000 {hi} 11000 0))
+    (rule (width 200) (clearance 200))
+  )
+  (library
+    (padstack Rect_Pad (shape (rect F.Cu -100 -100 100 100)) (attach off))
+    (image PAD (pin Rect_Pad 1 0 0))
+  )
+  (placement
+    (component PAD
+      (place P1 3000 {yc} front 0)
+      (place P2 17000 {yc} front 0)
+      {extra_place}
+    )
+  )
+  (network
+    (net N (pins P1-1 P2-1))
+    {extra_net}
+    (class default (rule (width 200) (clearance 200)))
+  )
+  (wiring)
+)
+"""
+
+
+def _route(text: str, rounds: int):
+    exact, _scale, _layers = route_dsn_board_exact(
+        parse_dsn(text), resolution=1, window_reroute=rounds, window_resolution=4,
+    )
+    return exact.result
+
+
+def test_the_board_grid_misses_the_slot_and_a_finer_window_finds_it():
+    plain = _route(_slot(), 0)
+    assert len(plain.failed) == 1
+    windowed = _route(_slot(), 1)
+    assert windowed.failed == []
+    assert windowed.wires  # and the copper it found is in the result
+
+
+def test_a_window_that_cannot_help_leaves_the_board_as_it_was():
+    plain = _route(_slot(gap=500, far=True), 0)  # too narrow for any grid
+    windowed = _route(_slot(gap=500, far=True), 2)
+    assert len(windowed.failed) == len(plain.failed) == 1
+    assert windowed.wires == plain.wires
+    assert windowed.vias == plain.vias
+
+
+def test_copper_outside_the_window_is_kept_as_it_was():
+    plain = _route(_slot(far=True), 0)
+    windowed = _route(_slot(far=True), 1)
+    assert windowed.failed == []
+    far_net = 2  # M, the second net in the DSN
+    assert windowed.wires[far_net] == plain.wires[far_net]
+
+
+def test_a_router_limited_to_some_nets_and_a_box_builds_only_those():
+    dsn = parse_dsn(_slot(far=True))
+    s = dsn.resolution.value
+    board = build_board(dsn)
+    whole = GridRouter(board, trace_width=round(_rule_width_dsn(dsn) * s),
+                       clearance=round(_rule_clearance_dsn(dsn) * s), orthogonal=True)
+    part = GridRouter(board, trace_width=round(_rule_width_dsn(dsn) * s),
+                      clearance=round(_rule_clearance_dsn(dsn) * s), orthogonal=True,
+                      only_nets=frozenset({2}), bounds=IntBox(0, -20000, 80000, 0))
+    assert {c.net_no for c in whole._build_connections()} == {1, 2}
+    assert {c.net_no for c in part._build_connections()} == {2}
+    assert part.cols * part.rows < whole.cols * whole.rows