feat(camera): round and feather the spotlight - #49

Why

The cutout was a clip-path: polygon: a hard-edged rectangle, and nothing else. Rounding the corners means emitting arcs by hand; softening the edge is not possible at all. On a pill button that reads as a crop rather than a highlight, and the one-pixel step to full scrim is the harshest part of the effect.

Same page, target and options, both sides calling argo's exported spotlight(). Bottom row crops the cutout's corner, upscaled nearest-neighbour so the interpolator cannot invent the soft edge in question.

Scrim luma across the cutout's left edge, at the shipped defaults:

before:  76, 76, 76, ... 76, 255                                    one-pixel step
after:   76, 77, 78, 80, 82, 84, 88, 93, 98, 105, 114, 124, 134,
         145, 157, 169, 180, 191, 202, 212, 221, 229, 235, 241,
         245, 248, 251                                              ramp over ~26px

What

• Every existing caller gets the softer edge without opting in. radius: 0, feather: 0 restores the old look exactly.
• Fades move to a cubic-bezier ease, so the arrival reads as a transition rather than a switch.
• Two mask attributes that fail quietly if dropped:
  • color-interpolation-filters="sRGB": SVG defaults to linearRGB, which ramps the scrim far too fast at the hole's edge.
  • Filter region widened to 200%: the default object bounding box clips the blur partway through its ramp on a small target. Holds while feather stays under about a third of the cutout's shorter side, which the option documents.
• The four defaults are passed into the page function rather than closed over, since Playwright serializes it. Written twice they could disagree.
• A NaN or a negative renders as a blank scrim with no hole, hence the clamps. padding is clamped for the first time here: on main a large negative value still leaves a hole, under the mask it removes one.

Known limitation: svg.innerHTML is a Trusted Types sink. On a page serving require-trusted-types-for 'script' the assignment throws, runCameraEffect swallows it, and no spotlight renders where main's style.cssText would have. Left as is because building the nodes costs the mask markup's readability. Happy to convert.

Test

• 774 tests on main