feat: Implement chunk-based Grid rendering for large grids (closes #123)

Adds a sub-grid system where grids larger than 64x64 cells are automatically
divided into 64x64 chunks, each with its own RenderTexture for incremental
rendering. This significantly improves performance for large grids by:

- Only re-rendering dirty chunks when cells are modified
- Caching rendered chunk textures between frames
- Viewport culling at the chunk level (skip invisible chunks entirely)

Implementation details:
- GridChunk class manages individual 64x64 cell regions with dirty tracking
- ChunkManager organizes chunks and routes cell access appropriately
- UIGrid::at() method transparently routes through chunks for large grids
- UIGrid::render() uses chunk-based blitting for large grids
- Compile-time CHUNK_SIZE (64) and CHUNK_THRESHOLD (64) constants
- Small grids (<= 64x64) continue to use flat storage (no regression)

Benchmark results show ~2x improvement in base layer render time for 100x100
grids (0.45ms -> 0.22ms) due to chunk caching.

Note: Dynamic layers (#147) still use full-grid textures; extending chunk
system to layers is tracked separately as #150.

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

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

src/UIGrid.h

@@ -21,6 +21,7 @@
 #include "UIDrawable.h"
 #include "UIBase.h"
 #include "GridLayers.h"
+#include "GridChunk.h"
 
 class UIGrid: public UIDrawable
 {
@@ -75,7 +76,15 @@ public:
     std::shared_ptr<PyTexture> getTexture();
     sf::Sprite sprite, output;
     sf::RenderTexture renderTexture;
+
+    // #123 - Chunk-based storage for large grid support
+    std::unique_ptr<ChunkManager> chunk_manager;
+    // Legacy flat storage (kept for small grids or compatibility)
     std::vector<UIGridPoint> points;
+    // Use chunks for grids larger than this threshold
+    static constexpr int CHUNK_THRESHOLD = 64;
+    bool use_chunks;
+
     std::shared_ptr<std::list<std::shared_ptr<UIEntity>>> entities;
 
     // UIDrawable children collection (speech bubbles, effects, overlays, etc.)