Add shared orthogonal shove primitives module

Factor the exact router's shove geometry into route/shove.py: the
conflicting-segment finder and displacement ladder are re-exported from
exact_router (kept byte-unchanged so its shove count and geometry stay
identical), and a room-oriented trace placement helper is added that
returns the inserted tile owner ids for a journaled rollback.

src/freeroute/route/shove.py

@@ -0,0 +1,124 @@
+"""Shared orthogonal shove primitives for the exact and room routers.
+
+The exact-geometry router (:mod:`freeroute.route.exact_router`) grew the first
+shove implementation; the continuous room router reuses its geometry so both
+tracks agree on how a blocking trace's conflicting segment is found, how the
+perpendicular displacement ladder is generated, and how a reshaped trace is
+verified before it is committed.
+
+To keep the exact track byte-for-byte identical (its tests assert an exact shove
+count and geometry), the two low-level geometry helpers still live in
+``exact_router`` and are re-exported here rather than moved. This module adds the
+room-specific placement helper, which returns the inserted tile owner ids: the
+room engine mutates already-committed nets, so it needs those ids for a real
+journaled rollback (the exact track's rollback is a no-op because it only ever
+verifies a shoved trace in isolation).
+"""
+
+from __future__ import annotations
+
+from collections.abc import Iterator
+
+from freeroute.board.search_tree import ShapeSearchTree, TreeShape
+from freeroute.geometry import IntBox, IntPoint, Polyline, PolylineShape
+
+from .exact_router import _conflicting_segment as conflicting_segment
+
+__all__ = [
+    "conflicting_segment",
+    "trace_tiles",
+    "shove_displacements",
+    "place_trace_owners",
+    "force_place_owners",
+]
+
+
+def trace_tiles(corners: list[IntPoint], half_width: int) -> list[IntBox]:
+    """The copper tiles of the trace centreline ``corners`` at ``half_width``."""
+    return PolylineShape(Polyline(corners), half_width).tiles()
+
+
+def shove_displacements(
+    corners: list[IntPoint],
+    seg_index: int,
+    horizontal: bool,
+    d_corners: list[IntPoint],
+    half_width: int,
+    clearance: int,
+) -> list[tuple[int, int]]:
+    """The perpendicular displacement ladder for shoving segment ``seg_index``.
+
+    Mirrors the exact router's ladder: displace the conflicting segment away
+    from the crossing net at ``d_corners`` (its straight copper) by just enough
+    to clear it (``2*half_width + clearance`` centre-to-centre), then step out in
+    ``half_width + clearance`` increments. The "away" side is tried first (past
+    the far edge of the crossing net), then the near side, so the search is
+    deterministic and integer-only.
+    """
+    d0, d1 = d_corners[0], d_corners[1]
+    margin = 2 * half_width + clearance
+    fine = half_width + clearance
+    ref = corners[seg_index]
+    out: list[tuple[int, int]] = []
+    if horizontal:  # displace in y to clear the crossing net's y-span
+        lo, hi = min(d0.y, d1.y), max(d0.y, d1.y)
+        for extra in range(6):
+            out.append((0, (hi + margin) - ref.y + extra * fine))
+            out.append((0, (lo - margin) - ref.y - extra * fine))
+    else:  # displace in x
+        lo, hi = min(d0.x, d1.x), max(d0.x, d1.x)
+        for extra in range(6):
+            out.append(((hi + margin) - ref.x + extra * fine, 0))
+            out.append(((lo - margin) - ref.x - extra * fine, 0))
+    return out
+
+
+def place_trace_owners(
+    net_no: int,
+    layer: int,
+    corners: list[IntPoint],
+    tree: ShapeSearchTree,
+    owner: Iterator[int],
+    half_width: int,
+    clearance: int,
+    outline: IntBox,
+) -> list[int] | None:
+    """Verify a trace's copper and insert it, returning the inserted owner ids.
+
+    The trace is accepted only if every tile stays inside ``outline`` and clears
+    every other net exactly (via ``tree.clearance_conflict``). On rejection
+    nothing is inserted and ``None`` is returned.
+    """
+    boxes = PolylineShape(Polyline(corners), half_width).tiles()
+    for box in boxes:
+        if not outline.contains_box(box):
+            return None
+        if tree.clearance_conflict(box, net_no, layer, clearance):
+            return None
+    owners: list[int] = []
+    for box in boxes:
+        oid = next(owner)
+        tree.insert(TreeShape(oid, net_no, frozenset({layer}), box, routed=True))
+        owners.append(oid)
+    return owners
+
+
+def force_place_owners(
+    net_no: int,
+    layer: int,
+    corners: list[IntPoint],
+    tree: ShapeSearchTree,
+    owner: Iterator[int],
+    half_width: int,
+) -> list[int]:
+    """Insert a trace's copper without any check, returning the owner ids.
+
+    Used only to restore an already-verified original trace during rollback: its
+    tiles were clean before the attempt, so no re-verification is needed.
+    """
+    owners: list[int] = []
+    for box in PolylineShape(Polyline(corners), half_width).tiles():
+        oid = next(owner)
+        tree.insert(TreeShape(oid, net_no, frozenset({layer}), box, routed=True))
+        owners.append(oid)
+    return owners