geometry-overlay

The boolean-overlay engine for the geometry port: the segment intersection kernel, the turn graph, traversal, and the intersection / union / difference / sym_difference operations built on top of them.

Mirrors boost/geometry/algorithms/detail/overlay/. This is the largest single subsystem in the port; it is built in strict phases (see specs/phase_03-rust-port-implementation-plan-overlay.md):

• OVL1 — predicate kernel (predicate/): orientation, in-circle, segment-segment intersection, and the coordinate-range robustness gate. Robustness policy: exact input arithmetic, no rescale — see specs/overlay-robustness-decision.md.
• OVL2 — turn graph, OVL3 — traversal, OVL4 — output assembly, OVL5 — the four overlay free functions, then OVL6+ fan out (relate/is_valid/buffer/merge_elements).

Cartesian only for v1.