Support PostgreSQL numeric promotion and missing numeric operators

[alexeichhorn]

Context

dbkit supports same-type numeric +, -, and *, but does not model PostgreSQL's broader numeric promotion and operator matrix.

Scope

Mixed numeric promotion

Support PostgreSQL-valid pairs such as:

SMALLINT + INTEGER         → INTEGER
INTEGER + BIGINT           → BIGINT
REAL + DOUBLE PRECISION    → DOUBLE PRECISION

Apply the same base-result matrix to +, -, and * in both supported operand directions.

Division and remainder

Add typed / and % expression support with PostgreSQL's result types and error behavior.

Conditional numeric functions

Use the common numeric type for calls such as:

COALESCE(integer_value, bigint_value)
GREATEST(smallint_value, integer_value)
LEAST(real_value, double_value)

Small integer widening

Allow i16 / Option<i16> in interval::days, hours, and minutes, where PostgreSQL widens SMALLINT to the INTEGER constructor argument.

Requirements

• Preserve Option<Output> whenever any SQL-propagating operand is nullable.
• Reject numeric/non-numeric combinations at compile time.
• Centralize the promotion matrix rather than duplicating it per operator/function.
• Cover both operand directions and boundary types.

This is separate from #46, which corrects the already-supported SMALLINT op SMALLINT result, and #49, which covers aggregate SUM result types.