Add RGBW white blend mode

[neominik]

Summary

Fixes #866.

This adds a white blend mode for RGBW lights where incoming RGB values are converted to RGBW by extracting the shared white component:

W = min(R, G, B)
R = R - W
G = G - W
B = B - W

This is useful for RGBW strips whose white LED is neutral rather than clearly warm or cold. The light can still be controlled as RGB, while the physical white channel contributes brightness and improves color output.

The mode is controlled by the new white blend flag. It is separate from the existing warm-white emulation behavior, so existing flag 24 behavior is preserved.

Implementation notes

• Adds a new LED white blend flag.
• Applies the blend in the shared LED driver path.
• Keeps forced RGB mode behavior unchanged; forced RGB mode disables this path.
• Adds LED self-test coverage for RGBW white blend output.

Configuration

For an RGBW strip, map the physical neutral white LED to the normal W channel and enable flag 52.