ev-next-free finds the earliest free slot >= after for a duration within a
horizon, probing 'after' + busy-interval ends via the busy_in rule (ev-free?).
Finds gaps, skips too-short gaps, half-open at edges. 59/59 green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>