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.
Ports the trace-relevant subset of geometry/planar/{Polyline,PolylineShape,
LineSegment}: a Polyline is a trace centreline (integer corners); a
PolylineShape is that centreline swept by the half-width, returned as exact
convex IntBox tiles (one flush box per orthogonal segment plus a square at
each interior corner to cover the turn). Exact for orthogonal segments; a
conservative bounding box for a diagonal one (never under-reports an
overlap). 45-degree/IntOctagon caps are deferred.
