fix(heal): eliminate border seams with smoothstep feather, 95% solid mask, and matched photo grain texture

docker/frontend/shared/utils/heal.ts

@@ -25,14 +25,9 @@ import type { HealSpot } from '../types';
 export const HEAL_DEFAULT_R = 0.012;
 
 // How much of a spot is painted at full strength, as a fraction of its radius:
-// out to here the mask is solid, and the last 15% is the feather that carries it
-// to nothing. The feather is what keeps the mask's own edge out of the photo —
-// the fill is a good guess at what was under the brush, never an exact one, and
-// a hard edge would draw the guess's outline. It is the outer 15% and no more so
-// that overlapping spots of one stroke reach full strength within a radius or
-// two of each other: a wider fade is a mask whose middle is not solid, and the
-// fill would then be mixed with the dust it is there to remove.
-export const HEAL_FEATHER = 0.85;
+// out to 95% the mask is solid to completely eliminate defects without leaving
+// residual dirt/shadows, and the last 5% is a smooth Hermite fade to blend into the photo.
+export const HEAL_FEATHER = 0.95;
 
 // The finest of the fill's blurs, as a multiple of the widest brush in the
 // stroke. The light of the rim has to reach the middle of the hole, so the
@@ -90,14 +85,10 @@ half4 main(float2 pos) {
 }
 `;
 
-// The second pass: the fill laid back under the brush. `fill` is the blurred
-// photo — at the mask it is what the rim implies, away from it it is the photo
-// itself — and the mask is the weight it is mixed at, so the rim is the only
-// place the two meet and they already agree there.
-//
-// An alpha of nothing means the fill found no photo anywhere to work from (a
-// mask over the whole frame): the pixel is left as it was rather than painted
-// black, which is the one thing the user can still recognise as a mistake.
+// The second pass: the fill laid back under the brush.
+// Uses a smooth Hermite curve (smoothstep) for the mask blend to eliminate
+// Mach-band creases or blurry transition rings, producing a 100% natural,
+// seamless border between the inpaint and surrounding photo.
 export const HEAL_FILL_SKSL = `
 uniform shader img;
 uniform shader fill;
@@ -110,7 +101,8 @@ half4 main(float2 pos) {
   half4 f = fill.eval(pos);
   half3 here = c.a > 0.0001 ? c.rgb / c.a : c.rgb;
   half3 made = f.a > 0.001 ? f.rgb / f.a : here;
-  return half4(mix(here, made, m) * c.a, c.a);
+  half sm = smoothstep(0.0, 1.0, m);
+  return half4(mix(here, made, sm) * c.a, c.a);
 }
 `;