fix(blend): composite colors using their alpha values - #407

Fixes #281.

blend() reads both inputs with rgb(), discarding their alpha values. A transparent top color can therefore change the bottom color, and two transparent inputs can produce an opaque result.

Composite the existing RGB blend functions using source-over alpha, with the second color over the first. Handle transparent operands before evaluating the blend, and clamp the overlap color before weighting it. Also handle the white-backdrop/black-source burn endpoint, which currently throws on 0/0.

The existing opaque RGB results and clipping behavior are retained, except that the undefined burn endpoint now returns white. Translucent results can now include alpha in the default hex() output; the documentation includes examples. A NaN alpha set through the mutating setter is now rejected.

Validation:

• Node 20.20.2, pnpm 10.24.0: pnpm lint, pnpm test -- --run (2,582 tests), and pnpm build pass.
• The final blend tests produce 39 intended failures and 14 passing controls with the original production source; all 53 pass with the fix.
• pnpm run docs passes on Node 24.18.1/Linux.

Regression coverage includes all eight modes, transparent and partially transparent operands, direct mode methods, RGB rounding, input Color immutability, burn clipping, and alpha modified through the mutating setter. No dependencies or generated files change.