Re-express the point-in-area interval index on the shared RTree

[asinghvi17]

Stacked on #440 (base branch relateng-payload-tree) — the last of the three substrate-audit follow-ups.

SortedPackedIntervalRTree was a from-scratch packed tree that duplicated the shared bulk-loading substrate piece by piece: its midpoint sort (JTS NodeComparator) is exactly sort-tile-recursive in one dimension, and its flat per-level min/max arrays are the same bottom-up consecutive-run extent packing. With #440's payload-carrying RTree, the whole thing is expressible as a constructor call: the prepared point-in-area locator's segment index is now

RTree(STR(), segs; extents = y_intervals)   # 1-D (Y,) extents

queried through SpatialTreeInterface.depth_first_search with a closed [y, y] interval extent (the do-block callback form, so the RayCrossingCounter loop stays allocation-free). What's preserved and what changed:

• The 1-D stabbing contract is unchanged: the horizontal ray must visit every segment whose y-interval contains p.y regardless of x, so a 2-D index could not prune more without changing the ray-counting contract; closed-interval touch semantics match (Extents.intersects is closed).
• The midpoint-sorted layout is preserved by construction — STR's first (only) dimension sort is the midpoint sort, keeping the measured ~3× query win over ring order for this build-once-query-forever index.
• The tree shape changes from JTS's binary pairwise packing to the RTree default 16-ary fanout; query results are extent-checked and set-identical (the test suite compares against brute force).
• IntervalIndexedGeometry.isEmpty becomes index === nothing (an empty polygonal geometry contributes no segments), replacing the separate is_empty flag.

Net: −104 lines (the source file shrinks by ~145), and RelateNG's last private index structure now sits on the shared substrate.

Tests

The SortedPackedIntervalRTree testset is re-expressed as a 1-D stabbing testset over the new index shape (same cases: duplicates, point intervals, closed-interval touches, brute-force sweeps). Green on this branch: indexed point-in-area (incl. the 10k-vertex prepared-vs-unprepared differential), point locator, allocations + type stability, LibGEOS differential fuzz (500).

⚠️ The two long gate files (relate_ng.jl engine, xml_suite.jl JTS conformance) were repeatedly terminated externally while running in the background (the second engine attempt died ~19 min in, likely moments from passing); they passed on the base branch #440 and this branch touches only the prepared locator's index internals, which the fuzz + differential testsets exercise end-to-end. They still need one clean confirmation run before undrafting.

🤖 Generated with Claude Code

[asinghvi17]

Collapsed into #416 for whole-piece review — the relateng branch was fast-forwarded to this stack's tip, so #416 now contains these commits. This PR stays as a focused, reviewable diff of its layer (see the review map in #416).