Collapse the two parallel selection systems into one. The RegionSelect
tool (rect + lasso) now cuts the geometry along the region outline and
selects the resulting sub-pieces into the standard `Selection` ID-sets,
exactly like every other tool. The vestigial floating `RegionSelection`
(drag never wired; commit/delete/copy were stubbed) and all its plumbing
are removed, so Group, Convert-to-Movie-Clip, Delete, and Properties all
operate uniformly from lasso, rect, marquee, and click selections.
Region cutting is reworked onto a robust planar arrangement:
- Replace fragile incremental "split a fill by one cut edge" logic with
planar face re-tracing (`retrace_fills_after_cut` + `trace_faces`),
which correctly handles arbitrary holed/concave fills.
- `extract_subgraph` no longer frees vertices still referenced by kept
boundary edges (fixed Group leaving freed-but-referenced vertices that
a later alloc reused and corrupted).
- `split_fill_by_*` direction fix (was producing disconnected boundaries
rendered as stray diagonals).
- `fill_interior_point` (area-centroid + inward-step fallback) for
reliable inside/outside classification of non-convex pieces.
- Coincident-edge dedupe + degenerate-fill removal (edge-adjacent shapes
no longer make zero-area sliver fills).
- Dangling-edge pruning, near-coincident endpoint welding, induced-
subgraph expansion, and tracking of `split_edge` sub-edges, so
self-intersecting freehand lassos cut correctly.
Region-select capture is available behind LIGHTNINGBEAM_DUMP_REGION=1 for
turning a misbehaving cut into a deterministic test. Extensive regression
tests added in vector_graph/tests/region_cut_select.rs.
39978e59b3