Handle unknown/approximate birth time or sample an explicit interval

The predecessor explicit interval remains exact: five-minute samples across at most six civil hours. The additive birth_time workflow accepts either an unknown local date (complete date sampled every 30 minutes) or an approximate local time with symmetric 5–180 minute uncertainty in five-minute increments. It reports observed sign/house stability and transition brackets, never selects a representative time, and explicitly does not produce an exact natal chart or rectification; 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.