Add docs/schedule-api-contract.md (current state, time semantics, schedule
data model, validation rules, endpoints, sync behavior, System Name rules,
dynamic audio, backward compatibility, per-phase touchpoints, open items,
changelog) and docs/fixtures/*.json (daily, weekly, none, midnight-crossing,
adjacent, dynamic-audio, and legacy v2 payloads). Kept byte-identical with
the radiantwave repo.
