fix(photos): preserve tapered tips and dark live edge (#73 #74)

Lower the axial body threshold while retaining fringe rejection, minimize length-only tip inset, and make sheet sensitivity curves testable and conservative at zero.

frontend/src/worker/mask.worker.ts

@@ -1,32 +1,13 @@
 import { minAreaRect, slabBounds } from '../geometry/rect';
 import type { MaskParams, WorkerIn, WorkerOut } from '../capture/capture.types';
 import { decodeOrientedBitmap } from '../decodeOrientedBitmap';
+import { greenKeep, blackKeep, autoPickSheet } from '../mask/sheetKeep';
 
 let bitmap: ImageBitmap | null = null;
 let data: Uint8ClampedArray | null = null;
 let w = 0, h = 0;
 let current = 0; // highest job id issued by main; older replies are dropped
 
-// Keep slab pixels; remove the sheet. Higher sensitivity widens the sheet match.
-const greenKeep = (r: number, g: number, b: number, s: number): boolean => {
-  const t = s / 100;
-  const isSheet = g - Math.max(r, b) > 10 + 35 * t;
-  return !isSheet;
-};
-const blackKeep = (r: number, g: number, b: number, s: number): boolean => {
-  const lum = 0.2126 * r + 0.7152 * g + 0.0722 * b;
-  const isSheet = lum < 40 + 140 * (s / 100);
-  return !isSheet;
-};
-const autoPick = (samples: { r: number; g: number; b: number }[]): 'green' | 'black' => {
-  let gr = 0, gl = 0;
-  for (const p of samples) {
-    if (p.g - Math.max(p.r, p.b) > 20) gr++;
-    if (0.2126 * p.r + 0.7152 * p.g + 0.0722 * p.b < 70) gl++;
-  }
-  return gl >= gr ? 'black' : 'green';
-};
-
 function runMask(params: MaskParams, id: number): WorkerOut {
   if (!data) return { id, kind: 'failed', reason: 'no source decoded' };
   let sheet = params.sheet;
@@ -37,7 +18,7 @@ function runMask(params: MaskParams, id: number): WorkerOut {
         const i = (y * w + x) * 4;
         samples.push({ r: data[i], g: data[i + 1], b: data[i + 2] });
       }
-    sheet = autoPick(samples);
+    sheet = autoPickSheet(samples);
   }
   const keep = sheet === 'green' ? greenKeep : blackKeep;
   const s = Math.max(0, Math.min(100, params.sensitivity));