Make query execution statically compilable

[quinnj]

Summary

• replace runtime callback fields with a style-dispatched extension interface
• keep connection, statement, cursor, and row-decoding paths concretely typed
• make transaction callbacks generic so caller return types survive inference
• fully drain simple-query responses through ReadyForQuery, including notices and errors
• add JuliaC trim coverage for typed queries, transactions, and protocol draining

Dependencies

• expects the two-tier StructUtils work from Two-tier make: tier-0 field-table interpreter default, :hot opt-in, classic family deleted JuliaServices/StructUtils.jl#63 for a zero-error full application graph

Verification

• 823/823 Postgres assertions pass against a real Docker PostgreSQL instance with released StructUtils 2.8.2
• released-dependency trim workload matches its explicit 92-error, zero-warning baseline
• LeagueEasy typed backend graph compiles and links under --trim=safe with zero verifier errors using this commit and the companion StructUtils pin
• all 10 GitHub Actions jobs pass across Linux, macOS ARM, Windows, minimum Julia, prerelease Julia, MD5, SCRAM, and docs

Co-authored by Codex