test: cover colour, hit testing and the stateless components

[meszmate]

Item 4 from the review — 64 of 88 source files had no tests. This is a first pass over the ones where behaviour is well-defined and bugs are cheap to miss.

44 tests across three files, covering style/color.zig, input/hitbox.zig, and the paginator, progress bar, spinner and keybinding map.

It found a real bug

MouseState.update checked the boundary crossing before the wheel:

if (inside and !was_hover) return .enter;
...
if (event.button == .wheel_up and inside) return .scroll_up;

A trackpad reports movement and scrolling in the same event, so a scroll that also moves the pointer into a region returned .enter and the scroll was dropped. In practice the first scroll into any scrollable region was being lost.

The wheel is now checked first. The crossing is not lost — the caller can still see it as state.hover.

What the tests pin down

Behaviour that is easy to break later, and that I checked against the implementation rather than assuming:

• hit boxes exclude their far edge, and saturate rather than wrapping at the u16 coordinate limit
• overlaps is symmetric and excludes merely-touching edges
• the full press → release → click cycle, and that a release outside does not click
• the 256-colour grayscale ramp stays monotonic; the cube's corners are black and white
• contrast ratio hits exactly 21:1 for black on white and 1:1 for a colour against itself
• NO_COLOR overrides COLORTERM=truecolor
• a paginator's partial last page — endIndex must not run past the item count
• the progress head marker disappears exactly when the bar is full

Two things worth knowing, found while writing these

• Progress.percent() returns 0–100, not 0–1. The tests now say so, since the name reads either way.
• Component view functions allocate scratch strings and never free them, by design — they expect the frame arena. That makes them leak under testing.allocator, so these tests hand them an arena. Worth documenting on the component contract.

zig build test and zig build clean on 0.16.0. Verified the hitbox test fails against the current code.