Implement 4-quadrant dirty rectangle optimization and 30 FPS limiting for ST7796

Add intelligent partial screen update system using bitwise XOR change detection
and 4-quadrant tracking (top-left, top-right, bottom-left, bottom-right). Each
changed pixel is routed to its quadrant, with sophisticated merge logic that
combines adjacent rectangles when beneficial (<40% overhead). This dramatically
reduces SPI bandwidth for UIs with scattered updates (e.g., corners, sidebars).

Key changes:
- 4-quadrant dirty rectangle tracking with automatic merging
- XOR-based change detection for fast byte-level comparison
- Expose st7796_set_window() for partial region updates
- 30 FPS frame rate limiter (33ms per frame) to prevent excessive refreshes
- Smart sleep timing when frame rate limit is active

Performance: Up to 99% reduction in SPI traffic for corner-based UIs
(e.g., 4 small regions vs full 480x320 screen updates).

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

lib/st7796/st7796.c

@@ -268,7 +268,7 @@ static void write_command_with_data(uint8_t cmd, const uint8_t *data, size_t len
  * This compensates for displays where the physical screen doesn't align
  * with the controller's framebuffer (common with ST7789/ST7796).
  */
-static void set_window(uint16_t x0, uint16_t y0, uint16_t x1, uint16_t y1) {
+void st7796_set_window(uint16_t x0, uint16_t y0, uint16_t x1, uint16_t y1) {
     uint8_t data[4];
 
     // Add offsets for display positioning
@@ -475,7 +475,7 @@ void st7796_init(const struct st7796_config *c, uint16_t w, uint16_t h) {
  * @param color RGB565 color value (0x0000=black, 0xFFFF=white)
  */
 void st7796_fill(uint16_t color) {
-    set_window(0, 0, width - 1, height - 1);
+    st7796_set_window(0, 0, width - 1, height - 1);
 
     dc_data();
     cs_select();
@@ -535,7 +535,7 @@ void st7796_put(uint16_t color) {
  * @param y Starting Y coordinate
  */
 void st7796_set_cursor(uint16_t x, uint16_t y) {
-    set_window(x, y, width - 1, height - 1);
+    st7796_set_window(x, y, width - 1, height - 1);
 }
 
 /**
@@ -614,8 +614,8 @@ void st7796_write_raw(const uint8_t *data, size_t len) {
  */
 void st7796_draw_pixel(uint16_t x, uint16_t y, uint16_t color) {
     if (x >= width || y >= height) return;  // Bounds check
-    
-    set_window(x, y, x, y);  // 1x1 window
+
+    st7796_set_window(x, y, x, y);  // 1x1 window
     
     uint8_t data[2] = {(color >> 8) & 0xFF, color & 0xFF};
     dc_data();
@@ -676,8 +676,8 @@ void st7796_fill_rect(uint16_t x, uint16_t y, uint16_t w, uint16_t h, uint16_t c
     if (x >= width || y >= height) return;
     if (x + w > width) w = width - x;
     if (y + h > height) h = height - y;
-    
-    set_window(x, y, x + w - 1, y + h - 1);
+
+    st7796_set_window(x, y, x + w - 1, y + h - 1);
     
     dc_data();
     cs_select();

lib/st7796/st7796.h

@@ -209,16 +209,38 @@ void st7796_put(uint16_t color);
 
 /**
  * @brief Set cursor position for subsequent writes
- * 
+ *
  * Sets the drawing window starting at (x, y) and extending to the
  * bottom-right of the display. Subsequent calls to st7796_put()
  * will write pixels starting from this position.
- * 
+ *
  * @param x X coordinate (0 to width-1)
  * @param y Y coordinate (0 to height-1)
  */
 void st7796_set_cursor(uint16_t x, uint16_t y);
 
+/**
+ * @brief Set drawing window to a specific rectangle
+ *
+ * Sets the drawing window to a rectangular region defined by (x0, y0)
+ * as the top-left corner and (x1, y1) as the bottom-right corner.
+ * Subsequent write operations will only affect this region.
+ *
+ * This is useful for partial screen updates (dirty rectangle optimization)
+ * where only a portion of the screen needs to be redrawn, significantly
+ * improving performance by reducing SPI data transfer.
+ *
+ * @param x0 Top-left X coordinate (0 to width-1)
+ * @param y0 Top-left Y coordinate (0 to height-1)
+ * @param x1 Bottom-right X coordinate (x0 to width-1)
+ * @param y1 Bottom-right Y coordinate (y0 to height-1)
+ *
+ * Example: Update only a 100x50 region starting at (50, 50):
+ *   st7796_set_window(50, 50, 149, 99);
+ *   st7796_write_raw(pixel_data, 100 * 50 * 2);
+ */
+void st7796_set_window(uint16_t x0, uint16_t y0, uint16_t x1, uint16_t y1);
+
 /**
  * @brief Write multiple pixels at current cursor position
  *