Add shove_segment geometric primitive

Displaces one segment of a trace polyline perpendicular by a given
offset, keeping the trace orthogonal and its two pad endpoints fixed: a
jog corner is inserted when the moved segment is first or last, and
interior neighbours simply resize. Collinear/duplicate corners are
dropped. This is the foundation for shoving a trace aside to make room
for another. Unit-tested for single-segment, interior-segment, and
orthogonality-preservation cases.

tests/geometry/test_polyline.py

@@ -2,7 +2,14 @@
 
 from __future__ import annotations
 
-from freeroute.geometry import IntBox, IntPoint, Polyline, PolylineShape, segment_box
+from freeroute.geometry import (
+    IntBox,
+    IntPoint,
+    Polyline,
+    PolylineShape,
+    segment_box,
+    shove_segment,
+)
 
 
 def test_polyline_segments():
@@ -42,3 +49,33 @@ def test_polyline_shape_bounding_box():
 def test_diagonal_segment_box_is_conservative_bbox():
     box = segment_box(IntPoint(0, 0), IntPoint(50, 50), 10)
     assert box == IntBox(-10, -10, 60, 60)
+
+
+# --- shove primitive --------------------------------------------------------
+
+
+def test_shove_single_segment_inserts_jogs_and_keeps_pads():
+    # a horizontal pad-to-pad trace shoved up by 30 keeps its endpoints
+    corners = [IntPoint(0, 0), IntPoint(100, 0)]
+    out = shove_segment(corners, 0, 0, 30)
+    assert [(p.x, p.y) for p in out] == [(0, 0), (0, 30), (100, 30), (100, 0)]
+    assert out[0] == corners[0] and out[-1] == corners[1]  # pads fixed
+
+
+def test_shove_interior_segment_resizes_neighbours():
+    # the middle horizontal run of a U shoved up 20; verticals resize, pads fixed
+    corners = [IntPoint(0, 0), IntPoint(0, 50), IntPoint(100, 50), IntPoint(100, 0)]
+    out = shove_segment(corners, 1, 0, 20)
+    assert [(p.x, p.y) for p in out] == [(0, 0), (0, 70), (100, 70), (100, 0)]
+
+
+def test_shove_stays_orthogonal():
+    corners = [IntPoint(0, 0), IntPoint(0, 40), IntPoint(80, 40), IntPoint(80, 0)]
+    out = shove_segment(corners, 1, 0, 15)
+    for a, b in zip(out, out[1:], strict=False):
+        assert a.x == b.x or a.y == b.y  # every segment axis-aligned
+
+
+def test_shove_out_of_range_is_noop():
+    corners = [IntPoint(0, 0), IntPoint(100, 0)]
+    assert shove_segment(corners, 5, 0, 30) == corners