fix: recalibrate enhancement scoring with dead zones

Contrast score now uses linear stdevLum/1.2 centered at 50 (was
miscalibrated 25-75 range centered at 75). Corrections use dead zones
(score 40-60 = zero) so well-exposed images get near-zero adjustments
instead of being darkened.

packages/image-engine/src/operations/auto-enhance.ts

@@ -119,9 +119,8 @@ function computeScores(
 ): AnalysisScores {
   const exposureScore = clamp(Math.round((meanLum / 255) * 100), 0, 100);
 
-  const idealStdev = 60;
-  const contrastDeviation = Math.abs(stdevLum - idealStdev) / idealStdev;
-  const contrastScore = clamp(Math.round((1 - contrastDeviation) * 50 + 25), 0, 100);
+  // Linear mapping: stdev ~60 = score 50. Higher stdev = higher contrast.
+  const contrastScore = clamp(Math.round(stdevLum / 1.2), 0, 100);
 
   const meanR = rCh.mean;
   const meanG = gCh.mean;
@@ -146,9 +145,9 @@ function computeScores(
 }
 
 function computeCorrections(scores: AnalysisScores): CorrectionParams {
-  const brightness = clamp(Math.round((50 - scores.exposure) * 1.2), -60, 60);
-  const contrast = clamp(Math.round((50 - scores.contrast) * 0.8), -40, 40);
-  const temperature = clamp(Math.round((50 - scores.whiteBalance) * 0.5), -30, 30);
+  const brightness = deadZoneCorrection(scores.exposure, 40, 60, 0.8);
+  const contrast = deadZoneCorrection(scores.contrast, 40, 60, 0.6);
+  const temperature = deadZoneCorrection(scores.whiteBalance, 40, 60, 0.5);
 
   const saturation =
     scores.saturation < 40
@@ -165,6 +164,16 @@ function computeCorrections(scores: AnalysisScores): CorrectionParams {
   return { brightness, contrast, temperature, saturation, sharpness, denoise };
 }
 
+/**
+ * Scores inside [lo, hi] produce zero correction.
+ * Scores outside scale from the dead zone edge, not from 50.
+ */
+function deadZoneCorrection(score: number, lo: number, hi: number, factor: number): number {
+  if (score >= lo && score <= hi) return 0;
+  if (score < lo) return clamp(Math.round((lo - score) * factor), 0, 60);
+  return clamp(Math.round((hi - score) * factor), -60, 0);
+}
+
 function detectIssues(scores: AnalysisScores): string[] {
   const issues: string[] = [];
   if (scores.exposure < 35) issues.push("underexposed");

tests/unit/auto-enhance.test.ts

@@ -84,6 +84,40 @@ describe("analyzeImage", () => {
     const result = await analyzeImage(darkBuffer);
     expect(result.suggestedMode).toBe("low-light");
   });
+
+  it("produces contrast score near 50 for a typical well-exposed image", async () => {
+    // A gradient image has stdev ~60, which should score ~50
+    const gradientBuffer = await sharp(
+      Buffer.from(
+        Array.from({ length: 100 * 100 * 3 }, (_, i) => Math.floor(((i % 300) * 255) / 300)),
+      ),
+      { raw: { width: 100, height: 100, channels: 3 } },
+    )
+      .png()
+      .toBuffer();
+
+    const result = await analyzeImage(gradientBuffer);
+    expect(result.scores.contrast).toBeGreaterThanOrEqual(35);
+    expect(result.scores.contrast).toBeLessThanOrEqual(65);
+  });
+
+  it("produces near-zero corrections for well-exposed images (dead zone)", async () => {
+    // Wide spread centered at 128 gives stdev ~60 (contrast ~50) and mean ~128 (exposure ~50).
+    // Both scores land inside the [40,60] dead zone, so corrections should be zero.
+    const midGrayBuffer = await sharp(
+      Buffer.from(
+        Array.from({ length: 100 * 100 * 3 }, (_, i) => 24 + Math.floor(Math.random() * 208)),
+      ),
+      { raw: { width: 100, height: 100, channels: 3 } },
+    )
+      .png()
+      .toBuffer();
+
+    const result = await analyzeImage(midGrayBuffer);
+    // Corrections should be zero or near-zero in the dead zone
+    expect(Math.abs(result.corrections.brightness)).toBeLessThanOrEqual(5);
+    expect(Math.abs(result.corrections.contrast)).toBeLessThanOrEqual(5);
+  });
 });
 
 describe("scaleCorrections", () => {