Temporal and decimal codecs for SQLite and MySQL #90

Signal

We want explicit, documented codecs for time and money on SQLite and MySQL:

• timestamp: DateTime.Utc. Storage is chosen per table: ISO-8601 text, or integer epoch milliseconds.
• date: a calendar date as YYYY-MM-DD text with no time zone.
• decimal(p, s): BigDecimal, stored as text on SQLite. It is never a float.

Why (MyRemotely)

• 274 DateTime columns and 5 Decimal columns.
• ADR-0019 in our app: a time tied to a unit uses that unit's time zone, and a calendar date has no zone. The codec must never shift a calendar date.
• SQLite has no native types for these. A wrong default here corrupts data quietly during the move.

Part of #52: the CAFE-stack roadmap signal from MyRemotely. This shows the direction we want, and it is not a demand. We can send a PR if you accept the idea.