Prune implicit machinery flagged by the substrate audit

[asinghvi17]

Stacked on #438 (base branch relateng-shared-substrate). The four small cleanups from the substrate audit; each is its own commit.

• Merge the byte-identical Planar/Spherical twin methods. _select_edge_set_accelerator and the AutoAccelerator arm of _build_prepared_edge_index now dispatch on Union{Planar, Spherical} (the manifolds with a segment-extent kernel). Also fixes the AutoAccelerator docstring, which still claimed non-Planar manifolds never take the tree path.

• Name the relateng tree accelerator DoubleNaturalTree. The tree path builds a NaturalIndex over each side's segment extents and traverses the two simultaneously — exactly what clipping's existing DoubleNaturalTree names. AutoAccelerator selected (and the docs advertised) DoubleSTRtree, which builds no STRtree here.

• Remove the lazy point-in-area auto-index. Unprepared locate_on_polygonal counted queries per polygonal element and silently switched to a lazily built IndexedPointInAreaLocator on the 9th — an implicit adaptive index deviating from Java, which keys indexing purely on isPrepared. Restored the Java rule: unprepared mode scans the rings on every query; repeated point location against one geometry is what prepare (the explicit escape hatch) is for. This orphans sort_leaves = false, so SortedPackedIntervalRTree always midpoint-sorts as JTS does (the measured sorted-vs-ring-order tradeoff stays recorded in its docstring), and poly_query_count / _LAZY_INDEX_QUERY_THRESHOLD are deleted.

  For the record, the removed heuristic's rationale: an unsorted index build cost ~10–13 ring scans (Natural Earth coastlines), so switching at 8 queries had worst-case regret of about one build. If unprepared multi-element relates ever show up hot, prepare covers it explicitly.

• Delete the unused is_known_entry. As ported it could never return false (matrix entries start at DIM_FALSE and only increase, so DIM_UNKNOWN never appears); a comment keeps the JTS-parity gap explained.

Tests

Full RelateNG suite green (JTS XML conformance, engine, LibGEOS differential fuzz, spherical conformance + end-to-end, allocations + type stability). The lazy-index testset now asserts the unprepared locator never builds an index; the prepared-vs-unprepared differential is unchanged.

🤖 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).