perf: prepare the lighting-corrected image once per photo

Threshold, click, scale and margin changes now reuse it, so each update
skips the ~200 ms grayscale and lighting fit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/detect.test.ts

@@ -1,5 +1,5 @@
 import { beforeAll, describe, expect, it } from 'vitest';
-import { detect, otsuThreshold } from '../src/lib/detect';
+import { detect, detectPrepared, otsuThreshold, prepareImage, preparedThreshold } from '../src/lib/detect';
 import { bounds } from '../src/lib/geometry';
 import { loadOpenCV, type CV } from '../src/lib/opencv-loader';
 import type { DetectParams } from '../src/lib/types';
@@ -114,4 +114,19 @@ describe('detect', () => {
     const framed = grayImage(400, 300, (x, y) => (inRect(x, y, 0, 60, 240, 260) || inRect(x, y, 330, 100, 340, 110) ? 220 : 30));
     expect(detect(cv, framed, params())).toBeNull();
   });
+
+  it('reuses a prepared image for repeated detections', () => {
+    const prepared = prepareImage(cv, plateScene);
+    try {
+      expect(preparedThreshold(cv, prepared)).toBe(otsuThreshold(cv, plateScene));
+      for (const marginPx of [0, -5]) {
+        const direct = detect(cv, plateScene, params({ marginPx }))!;
+        const reused = detectPrepared(cv, prepared, params({ marginPx }))!;
+        expect(bounds(reused.outer)).toEqual(bounds(direct.outer));
+        expect(reused.holes).toHaveLength(direct.holes.length);
+      }
+    } finally {
+      prepared.gray.delete();
+    }
+  });
 });