Add HDF (Highlight Diffusion Filter) to the FX tab

The built-in lens filter, simulated the way it is done in Photoshop: pull the
highlights out with a hard contrast curve (out = 2.5*in - 1.5, black below 0.6),
Gaussian-blur that selection, then Screen it back over the photo at reduced
opacity. Screen against a near-black pixel is a no-op, so areas with no
highlight within the blur radius come back byte-identical while lit areas
spread their light.

One 0..10 knob drives both the radius (4%..1.9% of the width, ~10..50px at
2560 wide) and the opacity (0.15..0.5), matching the recipe's 15-50px radius
range. The extraction composes on top of the recipe colour matrix so a
monochrome look glows white instead of leaking the original colours back.

Runs in the Skia export engine only; the P5 native path is not wired into the
app (dev parity probe only), so it is left alone for now.

src/utils/exportEngine.ts

@@ -300,6 +300,55 @@ export async function processAndExportPhoto(
       canvas.drawImage(skImage, 0, 0, paint);
     }
 
+    // 5b. Highlight Diffusion Filter (HDF): bloom on the bright areas only —
+    // the software twin of the lens' built-in diffusion filter, Photoshopped
+    // exactly as described: extract the highlights with a hard contrast curve,
+    // Gaussian-blur that selection (radius 15..50px), Screen it back over the
+    // photo at reduced opacity. Screen against a near-black pixel is a no-op, so
+    // shadows stay where they were while lit areas spread their light.
+    // One 0..10 knob drives both the blur radius and the opacity, and the radius
+    // is a fraction of the width so any source size blooms alike.
+    // ponytail: split into radius/amount knobs only if the 1D ramp reads coarse.
+    if ((adjustments.hdf ?? 0) > 0) {
+      const strength = (adjustments.hdf ?? 0) / 10;
+      const sigma = width * (0.004 + 0.015 * strength); // ~10..50px at 2560 wide
+      // Highlight extraction: out = 2.5*in - 1.5 -> black below 0.6, speculars
+      // saturate white. Composed on TOP of the recipe matrix so a monochrome look
+      // glows white instead of leaking the original colours back through.
+      const hiMatrix = [
+        2.5, 0, 0, 0, -1.5,
+        0, 2.5, 0, 0, -1.5,
+        0, 0, 2.5, 0, -1.5,
+        0, 0, 0, 1, 0,
+      ];
+      // The order is load-bearing and NOT the paint's: a paint color filter runs
+      // after the paint's image filter, i.e. the blur would smear the recipe
+      // matrix into the selection instead of pulling the highlights out first —
+      // measured as a flat +6/255 haze in the shadows with no glow on the
+      // speculars. Chain both as image filters instead (inner runs first).
+      const selection = Skia.ImageFilter.MakeColorFilter(
+        Skia.ColorFilter.MakeCompose(Skia.ColorFilter.MakeMatrix(hiMatrix), colorFilter),
+        null
+      );
+      const glowPaint = Skia.Paint();
+      glowPaint.setBlendMode(BlendMode.Screen);
+      glowPaint.setAlphaf(0.15 + 0.35 * strength);
+      glowPaint.setImageFilter(
+        Skia.ImageFilter.MakeCompose(
+          Skia.ImageFilter.MakeBlur(sigma, sigma, TileMode.Clamp, null),
+          selection
+        )
+      );
+      // Same source the base draw used, so the glow sits on the processed pixels.
+      if (paintShader) {
+        glowPaint.setShader(paintShader);
+        canvas.drawRect(Skia.XYWHRect(0, 0, width, height), glowPaint);
+      } else {
+        canvas.drawImage(skImage, 0, 0, glowPaint);
+      }
+    }
+
+
     // 6. Add Monochrome Grain Overlay
     if (adjustments.grain > 0) {
       const grainOpacity = adjustments.grain / 20; // up to 0.5 opacity

src/utils/paramDefs.ts

@@ -161,5 +161,16 @@ export const PARAM_DEFS: {
       get: (a) => a.grain,
       set: (v) => ({ grain: v }),
     },
+    {
+      key: 'hdf',
+      label: 'HDF',
+      min: 0,
+      max: 10,
+      step: 1,
+      defaultValue: 0,
+      display: String,
+      get: (a) => a.hdf ?? 0,
+      set: (v) => ({ hdf: v }),
+    },
   ],
 };