Remove YAGNI methods from performance systems

GridChunk: Remove getWorldBounds, markAllDirty, getVisibleChunks
- getWorldBounds: Chunk visibility handled by isVisible() instead
- markAllDirty: GridLayers uses per-cell markDirty() pattern
- getVisibleChunks: GridLayers computes visible range inline
- Keep dirtyChunks() for diagnostics

GridLayers: Remove getChunkCoords
- Trivial helper replaced by inline division throughout codebase

SpatialHash: Remove queryRect, totalEntities, cleanBucket
- queryRect: Exceeds #115 scope (only queryRadius required)
- totalEntities: Redundant with separate entity count tracking
- cleanBucket: Dead code - expired weak_ptrs cleaned during remove/update

All removals identified via cppcheck static analysis. Core functionality
of each system remains intact and actively used.

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

src/GridChunk.h

@@ -50,9 +50,6 @@ public:
     // Mark chunk as dirty
     void markDirty();
 
-    // Get pixel bounds of this chunk in world coordinates
-    sf::FloatRect getWorldBounds(int cell_width, int cell_height) const;
-
     // Check if chunk overlaps with viewport
     bool isVisible(float left_edge, float top_edge,
                    float right_edge, float bottom_edge) const;
@@ -90,13 +87,6 @@ public:
     UIGridPoint& at(int x, int y);
     const UIGridPoint& at(int x, int y) const;
 
-    // Mark all chunks dirty (for full rebuild)
-    void markAllDirty();
-
-    // Get chunks that overlap with viewport
-    std::vector<GridChunk*> getVisibleChunks(float left_edge, float top_edge,
-                                              float right_edge, float bottom_edge);
-
     // Resize grid (rebuilds chunks)
     void resize(int new_grid_x, int new_grid_y);