fix(Color): blueF() to return correct channel and tighten validation/docs

[joshtrichards]

Summary

This fixes OCP\Color::blueF() to return the correct channel and cleans up OCP\Color to make its API contract and behavior clearer.

• fix blueF() to return the blue - not green - channel
• clarify mixPalette() and alphaBlending() docs so they match actual current behavior
• add typed promoted properties
• dd input validation for channel ranges, palette steps, and blend opacity
• remove the unused legacy OC\Color class
• refactor internal helper naming for readability

Notes:

• we may want to consider switching to/using round() instead of raw int truncation for better interpolation quality, but I didn't want to risk blindly introducing that change in this public API
• mixPalette() behavior is intentionally unchanged: it returns a list that starts with $color1 and progresses toward $color2, excluding $color2 (now documented accurately!)
• alphaBlending() behavior is intentionally unchanged; this change documents and validates the existing contract
• stricter typing/validation may reject invalid inputs that were previously tolerated, but this should primarily surface already-invalid call paths
• validated against current avatar usage and updated the related test accordingly

P.S. I spent an embarrassing amount of time on this tiny class. 😆

TODO

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[joshtrichards]

Aww, crap. Forgot to update+commit the autoloader change. I'll do it later when I can pull the branch down (unless someone else beats me to it).