Support pgvector element-wise arithmetic

[alexeichhorn]

Context

pgvector provides element-wise vector arithmetic, while dbkit currently exposes distance/similarity operations only.

Supported pgvector operators include:

• vector + vector — element-wise addition
• vector - vector — element-wise subtraction
• vector * vector — element-wise multiplication

Expected behavior

• Support +, -, and * for PgVector<N> expressions.
• Return PgVector<N> for required operands and Option<PgVector<N>> when either operand is nullable.
• Require matching const dimensions at compile time.
• Reject vector/scalar and mismatched-dimension pairs.
• Cover column, literal, and computed-expression operands in both required/nullable combinations.
• Verify projected results against pgvector integration tests.

pgvector vector operators