Limit capture by samples per point, not by pixel width

[reesoousa]

Problem

The capture width is capped at a fixed number of pixels:

let scale = min(screen.backingScaleFactor, 2400 / area.width)

A fixed pixel budget spread over a larger display means fewer samples per point, so the fold is softer on a bigger MacBook than on a smaller one with the same chip behind it:

Display	Logical width	Capture	Samples per point
14 inch	1512 pt	2400 px	1.59
16 inch	1728 pt	2400 px	1.39

This turned up from a 16 inch owner reporting the effect looked worse than on a 14 inch, which is what sent me to this line.

Change

let scale = min(screen.backingScaleFactor, 2800 / area.width, 1.6)

Cap the ratio rather than the absolute width, so sharpness is the same on every built in display, and keep an absolute guard rail for any future larger panel.

1.6 is deliberate: it is where the old limit already landed on a 1512 point display, so the smaller machine is unchanged in practice and there is no regression to trade for the fix. A 1728 point display goes from 2400 to 2764 pixels wide, about a third more fragments through the blur chain, on hardware in the same family as the smaller machine that already handles it.

Why nothing else moves

Everything downstream of this line is already proportional, so the one line is the whole fix:

• blur sigmas are derived from the small texture width ($0 * smallWidth / 786), so they scale with the capture and stay the same fraction of the screen
• Fold.metal works entirely in normalized coordinates; its constants are proportions, not pixels
• the fold geometry is expressed as percentages of the frame

Testing

Built with make build on an M4 MacBook Pro, macOS 26.6.2, where the change is a no op by design (1.587 to 1.600). The larger display case is reported rather than measured by me: I do not have a 16 inch to hand, so I am relying on the arithmetic above plus a user report, and I would rather say so than imply I verified it on both panels.

I could not run npm run check locally: the swift-format shipped with the Command Line Tools cannot read this repo's configuration schema, so CI lint is the authority on formatting here.

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