Build deterministic model-ready natal context

Builds a model-neutral context from the existing fact-linked natal report. Facts retain stable IDs and API source paths; the calculation profile declares ephemeris, frame, zodiac, ayanamsa, houses, and pinned tzdb. A deterministic SHA-256 binds normalized input, profile, and facts. Optional JSON, XML, and Markdown artifacts contain no prompt, model instruction, generated interpretation, prediction, diagnosis, or advice. One standard quota unit per admitted request; failed or timed-out admitted attempts are not refunded. Historical mode uses an explicit BCE/CE date object and UT1 conventions. Its response is the versioned historical envelope, not the modern UTC envelope. Read result for calculation fields and metadata for limits and uncertainty.