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.
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