Fix luminance with non-integer Sass color channels

[cyphercodes]

Description

Rounds and clamps Sass RGB channel values before using them as indexes into the precomputed luminance lookup table.

Motivation & Context

Fixes #42447. Sass color operations can produce fractional channel values; using those values directly as nth() indexes can raise an invalid index error.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-{your_pr_number}--twbs-bootstrap.netlify.app/

Related issues

Fixes #42447

Verification

• npm run css-test
• npm run css-lint
• npm run lint (passes with existing TODO/FIXME warnings)
• npx stylelint scss/_functions.scss scss/tests/functions/_luminance.test.scss --cache --cache-location .cache/.stylelintcache
• git diff --check