feat(background): add the edges density ramp to pixel_grid

[LeadcodeDev]

Follow-up to #204.

What

pixel_grid shipped with radial — dense at the centre, thinning outwards. Porting it onto a real piece showed the opposite is what a background usually needs.

Measured on the reference texture, in a horizontal band clear of its window, cell density across the tenths of the frame:

10.9 · 6.5 · 0.8 · 0.2 · 0.2 · 0.2 · 0.2 · 0.9 · 7.8 · 8.8 %

Heavy at both edges, effectively empty through the middle 60 %. A vignette — which is what keeps the texture off whatever sits in the centre (here, a terminal window). None of the five existing ramps can express it: the one-way ramps are monotonic, and radial is exactly backwards.

I had first read the texture as "denser toward the right" from two crops, and built the demo on that. Two crops from opposite sides of the frame both look like a one-way ramp; only measuring the whole width shows the symmetry.

How

One arm, and the test asserts the relationship rather than describing it:

radial(p) + edges(p) == 1     // at every sampled point

plus the vignette's own shape — zero at the centre, ~1 at both left and right edges.

Docs

SKILL.md's ramp row now says what edges is for, and that radial is its inverse.

cargo test --workspace green, cargo fmt --check and cargo clippy --all-targets -- -D warnings clean.