Support additional date, time, and interval arithmetic

[alexeichhorn]

Context

dbkit currently supports timestamp/timestamptz + and - interval expressions. PostgreSQL exposes additional date, time, and interval arithmetic that dbkit rejects at compile time.

Candidate matrix

• INTERVAL ± INTERVAL → INTERVAL
• INTERVAL * DOUBLE PRECISION and the commutative multiplication form → INTERVAL
• INTERVAL / DOUBLE PRECISION → INTERVAL
• TIMESTAMP - TIMESTAMP → INTERVAL
• TIMESTAMPTZ - TIMESTAMPTZ → INTERVAL
• DATE ± INTEGER → DATE
• DATE ± INTERVAL → TIMESTAMP
• DATE - DATE → INTEGER
• DATE + TIME → TIMESTAMP
• TIME ± INTERVAL → TIME
• TIME - TIME → INTERVAL

Requirements

• Implement only PostgreSQL-defined operand directions and result types.
• Propagate nullable operands to Option<Output>.
• Add compile-fail coverage for invalid temporal pairs.
• Test calendar boundaries, negative intervals, and NULL operands where relevant.
• Keep implicit cross-type temporal comparisons and session-timezone-dependent comparison behavior out of scope.

PostgreSQL date/time operators