fix: optimize brush stroke handling and improve cursor preview

- Changed default VISION_BACKEND from CPU to GPU for better performance.
- Made `map_brush_style` public to allow external access.
- Updated `Engine` struct to use pooled buffers for stroke snapshots, reducing memory allocations during brush strokes.
- Introduced `CompositeSnapshot` for efficient background compositing without blocking the UI thread.
- Implemented `SendPtr` for safe raw pointer handling across threads.
- Enhanced `commit_pending_history` to recycle buffers and improve performance.
- Fixed cursor preview issues for procedural brushes by removing unnecessary preset synchronization.
- Added tests and documentation for new features and performance improvements.

hcie-engine-api/src/dynamic_loader.rs

@@ -7,9 +7,9 @@ use libloading::Library;
 
 /// Global vision backend preference mirrored into the dynamic `hcie-vision` plugin.
 /// 0 = CPU, 1 = GPU.  Stored here so it can be set before the plugin is loaded.
-static VISION_BACKEND: AtomicU8 = AtomicU8::new(0);
+static VISION_BACKEND: AtomicU8 = AtomicU8::new(1);
 
-fn map_brush_style(style: hcie_protocol::BrushStyle) -> hcie_brush_engine::BrushStyle {
+pub fn map_brush_style(style: hcie_protocol::BrushStyle) -> hcie_brush_engine::BrushStyle {
     match style {
         hcie_protocol::BrushStyle::Round => hcie_brush_engine::BrushStyle::Round,
         hcie_protocol::BrushStyle::Square => hcie_brush_engine::BrushStyle::Square,