Add separating-axis early-out to ConvexConvexSutherlandHodgman for disjoint inputs

[asinghvi17]

Fixes #408.

intersection(ConvexConvexSutherlandHodgman(...), a, b) previously ran the full Sutherland-Hodgman clip — allocating intermediate point buffers — even when the two convex polygons are disjoint, which is the dominant case in workloads like ConservativeRegridding.jl (~80% of spatial-tree candidate pairs).

Change

Adds a separating-axis early-out before any buffers are built, for both manifolds:

• Spherical: for each edge of either polygon, the edge's great-circle normal (robust_cross_product, hoisted out of the vertex loop) defines a candidate separating plane. If every vertex of the other polygon lies strictly on the negative side (semantics exactly matching spherical_orient < 0, same tolerance), the polygons are disjoint and the empty degenerate polygon is returned immediately.
• Planar: the classic SAT equivalent using Predicates.orient.

The check is only a sufficient condition: touching, shared-edge, and exactly-antipodal configurations (where vertices lie on the candidate great circles) fall through to the full clip, so results never change — only speed. Degenerate edges (e.g. the ring's closing point) are skipped, since robust_cross_product(a, a) returns an arbitrary orthogonal vector that must not be used as a candidate separating plane.

Type-level toggle

The early-out is selected at the type level via a new DisjointCheck enum carried as a second type parameter, defaulting to on:

ConvexConvexSutherlandHodgman()                                # ::ConvexConvexSutherlandHodgman{Planar, CheckDisjoint}
ConvexConvexSutherlandHodgman(Spherical(), SkipDisjointCheck)  # early-out disabled

Since the enum value is a type parameter, the branch is resolved at compile time. Existing dispatch on ConvexConvexSutherlandHodgman{Planar} etc. still works via partial instantiation, and the constructor validates the parameter is a DisjointCheck value.

Benchmarks (Chairmarks, min time, 1°×1° cells)

Case	SkipDisjointCheck (= before)	CheckDisjoint (default)
spherical disjoint	367 ns, 24 allocs	40 ns, 6 allocs (~10×)
planar disjoint	60 ns, 10 allocs	25 ns, 6 allocs (~2.4×)
spherical adjacent (shared edge)	383 ns	398 ns (falls through)
spherical overlapping	431 ns	495 ns (cost of the failed sweep)
planar overlapping	170 ns	166 ns (unchanged)

The remaining 6 allocations on the disjoint path are the returned degenerate polygon itself. For the ~80%-disjoint regridding mix described in the issue this is roughly a 3× overall speedup of the clipping step; workloads where pairs almost always overlap can opt out with SkipDisjointCheck.

Tests

• Direct predicate tests for both helpers: disjoint far/near/diagonal-gap pairs trigger in both argument orders; shared-edge, corner-touching, overlapping, and identical inputs do not (proving the conservative fall-through).
• End-to-end zero-area results for disjoint inputs in both argument orders, plus an exactly-antipodal spherical pair documenting that the early-out stays conservative while the full clip still returns empty.
• The DisjointCheck toggle: default parameter value, invalid-parameter ArgumentError, and identical results with the check disabled on both manifolds.

🤖 Generated with Claude Code

[asinghvi17]

This may not actually help in the use case I care about, so I'm going to leave it as a draft for now until I can establish whether it helps.