Executable decimal interchange arithmetic

This import provides Arithmetic.add, sub, mul, div, fma, sqrt, quantize, and the exact and explicit-direction integral-rounding variants, with all five rounding modes, preferred decimal cohorts, and default exception flags. It also includes Inspection.Runtime: sign operations, classification, all 22 comparison predicates, total ordering and quantum queries. It contains no proof-facet imports. Use Arithmetic.Basic for the value, range, error, signed-zero, and status theorems.

The operations consume and return Datum; BID and DPD remain representation choices at the codec boundary. Traps and a mutable exception environment are not part of this functional API.