docs: maintainability comments + low-hanging cleanup

- sample_bilinear_rgba: document alpha interpolation approach (M6)
- ROTATE_CURSOR_MAP: document ring structure and fallback (M7)
- rotate_sprite_expand_top_left: avoid cloning empty sprite (L1)
- RenderControlsPanel: reuse currentDeg for slider value, eliminating
  double Number.parseFloat (L2)
- TextBlockLayer: avoid rotate(-0deg) in number badge (L7)

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

Author: zachelnet

crates/koharu-app/src/renderer.rs

@@ -1058,7 +1058,7 @@ fn rotate_sprite_expand_top_left(src: &RgbaImage, angle_rad: f32) -> (RgbaImage,
     let src_w = src.width();
     let src_h = src.height();
     if src_w == 0 || src_h == 0 {
-        return (src.clone(), 0.0, 0.0);
+        return (RgbaImage::new(0, 0), 0.0, 0.0);
     }
 
     // Fast path: exact 90° multiples — direct integer pixel rearrangement,
@@ -1150,6 +1150,13 @@ fn rotate_point_top_left(x: f32, y: f32, cos: f32, sin: f32) -> (f32, f32) {
     (cos * x - sin * y, sin * x + cos * y)
 }
 
+/// Bilinear sample of an RGBA sprite.
+///
+/// Interpolates RGB in premultiplied-alpha space and alpha in straight-alpha
+/// space, then un-premultiplies once at the end. This is mathematically
+/// equivalent to the standard "premultiply all four channels, interpolate,
+/// un-premultiply" approach but avoids premultiplying alpha (which is
+/// invariant under premultiplication) through the interpolation step.
 fn sample_bilinear_rgba(src: &RgbaImage, x: f32, y: f32) -> Rgba<u8> {
     let max_x = src.width() as f32 - 1.0;
     let max_y = src.height() as f32 - 1.0;

ui/components/canvas/TextBlockLayer.tsx

@@ -24,7 +24,18 @@ type BoxGeometry = {
   height: number
 }
 
-/** Rotate a CSS resize-cursor name by the given degrees (snapped to 45° steps). */
+/**
+ * Maps CSS resize cursors through 45° rotation steps.
+ *
+ * Each entry is a ring of 8 cursor names at 0°, 45°, 90°, …, 315°.
+ * - Index 0: 0° (identity)
+ * - Index 1: 45°
+ * - Index 2: 90°
+ * - …
+ * - Index 7: 315°
+ *
+ * Unknown cursors fall back to the original name (see `rotateCursor`).
+ */
 const ROTATE_CURSOR_MAP: Record<string, readonly string[]> = {
   'ns-resize':   ['ns-resize',   'nesw-resize', 'ew-resize',   'nwse-resize', 'ns-resize',   'nesw-resize', 'ew-resize',   'nwse-resize'],
   'ew-resize':   ['ew-resize',   'nwse-resize', 'ns-resize',   'nesw-resize', 'ew-resize',   'nwse-resize', 'ns-resize',   'nesw-resize'],
@@ -281,7 +292,7 @@ function TextBlockItem({
         className={`pointer-events-none absolute -top-1.5 -left-1.5 flex h-4 w-4 items-center justify-center rounded-full text-[9px] font-semibold text-white shadow ${
           selected ? 'bg-primary' : 'bg-rose-400'
         }`}
-        style={{ transform: `rotate(${-(t.rotationDeg ?? 0)}deg)` }}
+        style={{ transform: `rotate(${t.rotationDeg ? -t.rotationDeg : 0}deg)` }}
       >
         {index + 1}
       </div>

ui/components/panels/RenderControlsPanel.tsx

@@ -846,11 +846,7 @@ export function RenderControlsPanel() {
             max={String(MAX_ROTATION_DEG)}
             step='1'
             disabled={!selectedNode}
-            value={
-              rotationDraft != null && Number.isFinite(Number.parseFloat(rotationDraft))
-                ? String(clampRotationDeg(Number.parseFloat(rotationDraft)))
-                : String(selectedNode?.transform.rotationDeg ?? 0)
-            }
+            value={String(currentDeg)}
             onChange={(event) => {
               setRotationDraft(event.target.value)
             }}