Carry payloads in RTree and dissolve the relate edge-index side-tables

[asinghvi17]

Stacked on #439 (base branch relateng-cleanups). The "payload-carrying tree" follow-up from the substrate audit, in two commits.

RTree carries the collection it indexes. Queries already return indices into the input collection, but the tree threw the collection away — so every consumer had to carry a parallel payload vector alongside the tree (JTS trees carry their items; the index-returning design needs the vector to live somewhere). It now lives on the tree as tree.data. Two supporting changes:

• An extents kwarg supplies precomputed extents (one per element, in order) for payload elements that carry no extent of their own, or extents computed in another coordinate space — the relate kernel computes segment extents over kernel-converted points, which GI.extent on the payload could never reproduce.
• Unsorted loading returns Base.OneTo from loadorder, so the constructor skips the identity permutation and aliases the extents vector as the leaf level. Zero copies — the same build cost as NaturalIndex, making RTree(Unsorted(), ...) a true natural index. (RTreeNode is reparametrized on the concrete tree type since RTree gained type parameters.)

The relate edge index dissolves its side-tables. _relate_edge_index(m, ss_list) now returns a natural-order RTree whose data is each leaf's (string index, segment index) owner, built from the flattened segment extents (or nothing for empty lists). The three dual-traversal sites map hits through tree.data[i] instead of a parallel owners vector, and PreparedEdgeIndex — a struct pairing exactly that tree with exactly that vector — dissolves into the tree itself: PreparedRelate.edge_tree is now the RTree or nothing.

This sets up the interval-tree follow-up: SortedPackedIntervalRTree is a midpoint-sorted packed tree over 1-D intervals carrying segment payloads, which is RTree(STR(), segs; extents = y_intervals) — STR in one dimension is the midpoint sort.

Tests

New FlexibleRTrees testset covers payload round-trips under reordering, the Unsorted zero-copy aliasing, collected non-vector input, and the extents/data length mismatch throw; the existing brute-force query sweeps now exercise the zero-copy Unsorted path. RelateNG: engine + prepared mode, edge intersector accelerator differentials, spherical end-to-end, JTS XML conformance, LibGEOS fuzz, and the allocation budget (256-vertex pairs exceed the tree threshold, so the budget covers the new build path) all green.

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[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).