ADR 0003: minimal public API

Status: accepted

The stable entry points are load_config, WavefrontSensor, and simulate. Users construct one configured sensor and call photon_rate, expose, expose_many, or expose_integrated with phase/OPD arrays. Configuration and wavefront are the only physical inputs to an exposure; a seed is an optional detector reproducibility control. Optical engines and detector adapters remain internal so new sensor types do not expand the top-level API.