Return f64 from pgvector scalar operations

[alexeichhorn]

Problem

pgvector distance and inner-product operations return PostgreSQL DOUBLE PRECISION, but dbkit models them as f32 / Option<f32>.

Affected helpers:

• l2_distance
• cosine_distance
• inner_product
• l1_distance
• inner_product_distance

Filtering and ordering can appear to work because PostgreSQL coerces thresholds. Selecting a result into the advertised Rust f32 can fail SQLx decoding because the database returns DOUBLE PRECISION.

Expected behavior

• Return Expr<f64> when both operands are required.
• Return Expr<Option<f64>> when either operand is nullable.
• Preserve vector dimension checking and current NULL propagation.
• Update explicit f32 thresholds, API assertions, and documentation to f64 where they consume these scalar results.
• Add required and nullable integration projections that decode the returned scalar.

pgvector vector function result types