Add SCREEN_DIM_15 (Off / 100 / 50 / 30 / 15). The value now needs 3 bits, so
repurpose the spare BIT_UNUSED_2 as BIT_SCREEN_DIM_2; existing codeplugs (bit 2
clear) keep their level, no reset.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>