Route constant-geometry spatial-relationship predicates to the TRTREE index

[estebanzimanyi]

eIntersects, eContains, eDisjoint and eTouches against a constant geometry are recognised by the TRTREE scan optimizer, which synthesizes the bounding-box && prefilter from the constant and probes the index (single-box or MEST). The bbox is a lossy superset, so the index scan reports supports_pushdown_type = false and DuckDB keeps the original predicate as an exact recheck filter above the scan, mirroring the lossy-index-always-rechecks contract of PostGIS GiST and MobilityDB's tspatial_supportfn. A parity block proves that a row whose bounding box overlaps the query polygon but does not actually intersect it is dropped, identically with and without the index and under MEST multi-entry. Stacked on #143; its commits show in the diff until it merges.

[estebanzimanyi]

Reviewer's quickstart — ~6 minutes

What this PR does in one sentence: when a spatial-rel predicate has a constant geometry on one side (e.g. eIntersects(tgeompoint, 'POLYGON(...)'::GEOMETRY)), routes the lookup through the TRTREE multi-entry index instead of scanning every row — proper index pushdown for the most common geofence query shape.

Files (4):

• src/index/rtree_optimize_scan.cpp — the scan-optimisation pass that recognises the constant-geometry pattern and rewrites the plan node.
• src/index/rtree_index_scan.cpp — the new constant-geometry-aware scan entry point.
• src/include/index/rtree_module.hpp — public declarations for the optimiser hook.
• test/sql/index/ — assertions that the EXPLAIN plan uses the index for the new pattern.

Verification:

CREATE INDEX rt ON trips USING TRTREE (traj);
EXPLAIN SELECT count(*) FROM trips WHERE eIntersects(traj, 'POLYGON(...)'::GEOMETRY);
-- Before this PR: SEQ_SCAN.  After: TRTREE_INDEX_SCAN.

Cross-link: stacked on #143 (MEST multi-entry indexing), which is in turn the foundation for this optimisation. Both will rebuild cleanly once #161 lands (Linux arm64 build).

Why it's safe to merge: the pre-existing scan path remains available for non-constant-geometry cases; this only adds a new index-aware branch.