Standalone data structure — no consumers yet. Flat-vector node
storage, leaf capacity 32 / max depth 16, O(1) remove/update via
handle locator map, out-of-root items kept in an overflow list that
surfaces on every query.
Foundation for replacing the O(N) entity scan in the wire tessellation
path (scene/mod.rs:1232) with a query_rect call.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
