Completes issue #11. Tagged.Scalar.timestampValue parses a !!timestamp
scalar under the YAML 1.1 grammar into the pure-Swift Tagged.Timestamp
(calendar components, timezone offset, secondsSinceEpoch via the
days-from-civil closed form; epoch test expectations measured against
Python's independent implementation). Tagged.Scalar.binaryValue base64
decodes a !!binary scalar into bytes, accepting unpadded and block
scalar wrapped content and rejecting corrupt input whole.
Yams hands back Foundation Date/Data; this package is mechanically
Foundation-free, so the bridges are caller-owned one-liners:
Date(timeIntervalSince1970:) and Data(_:). !!set and !!omap stay
skipped since Model.Mapping already preserves order and duplicates.
Closes#11
