fix(rgb): reject non-numeric channels

[xianjianlf2]

What changed

Reject non-numeric channels passed through explicit RGB mode instead of silently converting them to 0.

Why

As discussed in #356, clipping out-of-range numeric channels is intentional, but nonsensical values such as strings or null should not be accepted as RGB channels. This keeps numeric clipping intact while making invalid RGB input fail clearly.

Fixes #356

Validation

• npm test -- test/autodetect.test.js --run
• pre-commit ran npm run lint and full npm test successfully: 50 test files, 2521 tests

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Latest commit: 6b285a8

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