feat(scene): quadtree primitive for 2D spatial index (Phase 2.1, step 1)

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>

src/scene/mod.rs

@@ -14,6 +14,7 @@ pub mod object;
 pub mod paper_canvas;
 pub mod pipeline;
 pub mod properties;
+pub mod quadtree;
 mod render;
 mod selection;
 pub mod solid3d_tess;