fix(scale): clamp out-of-domain input before applying gamma

[apoorva-01]

Fixes #331.

chroma.scale('YlGn').domain([5,15]).gamma(1.2)(4) throws instead of clamping the out-of-domain value. With a fractional gamma, pow(t, _gamma) runs while t is still negative (t is -0.1 for input 4), and Math.pow(-0.1, 1.2) is NaN, which falls through to an invalid color. The limit(t, 0, 1) a few lines down handles the no-gamma path, but the gamma branch has already produced NaN by then.

The fix clamps t into [0,1] before the pow, reusing the limit helper that's already imported. Out-of-domain input now pins to the nearest endpoint, same as it does without a gamma. This matches @regorxxx's fork fix you said you'd look at, just using limit instead of an explicit if/else.

One call worth your eye: limit also clamps the high side, so an above-max input sits at the padded endpoint rather than the hard edge. With default padding that's identical to before and the full suite stays green, but if you'd rather scope the change strictly to the negative side that broke, pow(t < 0 ? 0 : t, _gamma) does only that.

Test in test/scales.test.js covers both ends. The below-domain case throws on main and passes with the fix.

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