feat: Implement SpatialHash for O(1) entity spatial queries (closes #115)

Add SpatialHash class for efficient spatial queries on entities:
- New SpatialHash.h/cpp with bucket-based spatial hashing
- Grid.entities_in_radius(x, y, radius) method for O(k) queries
- Automatic spatial hash updates on entity add/remove/move

Benchmark results at 2,000 entities:
- Single query: 16.2× faster (0.044ms → 0.003ms)
- N×N visibility: 104.8× faster (74ms → 1ms)

This enables efficient range queries for AI, visibility, and
collision detection without scanning all entities.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/UIGrid.h

@@ -22,6 +22,7 @@
 #include "UIBase.h"
 #include "GridLayers.h"
 #include "GridChunk.h"
+#include "SpatialHash.h"
 
 class UIGrid: public UIDrawable
 {
@@ -87,6 +88,9 @@ public:
 
     std::shared_ptr<std::list<std::shared_ptr<UIEntity>>> entities;
 
+    // Spatial hash for O(1) entity queries (#115)
+    SpatialHash spatial_hash;
+
     // UIDrawable children collection (speech bubbles, effects, overlays, etc.)
     std::shared_ptr<std::vector<std::shared_ptr<UIDrawable>>> children;
     bool children_need_sort = true;  // Dirty flag for z_index sorting
@@ -165,6 +169,7 @@ public:
     static PyObject* py_get_dijkstra_distance(PyUIGridObject* self, PyObject* args);
     static PyObject* py_get_dijkstra_path(PyUIGridObject* self, PyObject* args);
     static PyObject* py_compute_astar_path(PyUIGridObject* self, PyObject* args, PyObject* kwds);
+    static PyObject* py_entities_in_radius(PyUIGridObject* self, PyObject* args, PyObject* kwds);  // #115
     static PyMethodDef methods[];
     static PyGetSetDef getsetters[];
     static PyObject* get_entities(PyUIGridObject* self, void* closure);