perf: throttle rotation slider, add 90/180/270 fast path

- Range slider now buffers via rotationDraft and commits only on
  mouseUp, avoiding per-pixel applyOp/queueAutoRender calls during drag.
- rotate_sprite_expand_top_left detects exact 90° multiples and uses
  direct integer pixel rearrangement instead of bilinear resampling,
  avoiding a full per-pixel float pass + allocation for common angles.

Co-authored-by: DeepSeek V4 <deepseek@v4.ai>

Author: zachelnet

crates/koharu-app/src/renderer.rs

@@ -1045,6 +1045,45 @@ fn rotate_sprite_expand_top_left(src: &RgbaImage, angle_rad: f32) -> (RgbaImage,
         return (src.clone(), 0.0, 0.0);
     }
 
+    // Fast path: exact 90° multiples — direct integer pixel rearrangement,
+    // no resampling. The layout matches the bilinear path exactly.
+    let deg = angle_rad.to_degrees();
+    let remainder = deg % 90.0;
+    if remainder.abs() < 0.001 || (90.0 - remainder.abs()).abs() < 0.001 {
+        let normalized = ((deg % 360.0) + 360.0) % 360.0;
+        return match normalized.round() as i32 {
+            0 => (src.clone(), 0.0, 0.0),
+            90 => {
+                let mut dst = RgbaImage::new(src_h, src_w);
+                for y in 0..src_h {
+                    for x in 0..src_w {
+                        dst.put_pixel(src_h - 1 - y, x, *src.get_pixel(x, y));
+                    }
+                }
+                (dst, -(src_h as f32), 0.0)
+            }
+            180 => {
+                let mut dst = RgbaImage::new(src_w, src_h);
+                for y in 0..src_h {
+                    for x in 0..src_w {
+                        dst.put_pixel(src_w - 1 - x, src_h - 1 - y, *src.get_pixel(x, y));
+                    }
+                }
+                (dst, -(src_w as f32), -(src_h as f32))
+            }
+            270 => {
+                let mut dst = RgbaImage::new(src_h, src_w);
+                for y in 0..src_h {
+                    for x in 0..src_w {
+                        dst.put_pixel(y, src_w - 1 - x, *src.get_pixel(x, y));
+                    }
+                }
+                (dst, 0.0, -(src_w as f32))
+            }
+            _ => unreachable!(),
+        };
+    }
+
     let cos = angle_rad.cos();
     let sin = angle_rad.sin();
     let corners = [

ui/components/panels/RenderControlsPanel.tsx

@@ -829,9 +829,14 @@ export function RenderControlsPanel() {
             max={String(MAX_ROTATION_DEG)}
             step='1'
             disabled={!selectedNode}
-            value={selectedNode?.transform.rotationDeg ?? 0}
+            value={rotationDraft ?? String(selectedNode?.transform.rotationDeg ?? 0)}
             onChange={(event) => {
-              const parsed = Number.parseFloat(event.target.value)
+              setRotationDraft(event.target.value)
+            }}
+            onMouseUp={() => {
+              if (rotationDraft == null) return
+              const parsed = Number.parseFloat(rotationDraft)
+              setRotationDraft(null)
               if (!Number.isFinite(parsed)) return
               void updateSelectedRotation(parsed)
             }}