fix(background): make pixel_grid's twinkle reach both ends of the range

[LeadcodeDev]

What

pixel_grid's twinkle oscillated the cell alpha over 0.10..1.00, so a cell never went out. A lattice whose cells only dip to a tenth still reads as a fixed field of permanent dots — just dimmer — which is not what an animated texture is for.

- let a = 0.55 + 0.45 * (t * 1.6 + phase).sin();   // 0.10 .. 1.00
+ let a = 0.5  + 0.5  * (t * 1.6 + phase).sin();   // 0.00 .. 1.00

The phase still comes from each cell's own hash, or the whole field blinks in unison.

Test

It pins the range, not the fact that something moves: 400 sampled instants, floor under 0.01, ceiling over 0.99. "It animates" and "it reaches zero" are different claims, and only the second one was asked for.

Note

This should have shipped with #205 — I said it had, and it had not: the change was still uncommitted in my working tree when that branch was pushed. Caught by a git diff when the branch refused to switch.

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