Adds an in-process startup-handshake test using net.Pipe so we
can pin the protocol envelope (StartupMessage → AuthenticationOk
→ ParameterStatus×N → BackendKeyData → ReadyForQuery) without
binding a real TCP port. Runs in <1ms; safe for CI.
The PRG-dispatch path (runSQL → FIVE_SQL → row encoding) is
already covered manually by spinning a `five run` of
`pg_server_start(":15432")` and connecting with pgx — that flow
verified post-MVP that a real PostgreSQL client receives
`{ONE (INT4), GREET (TEXT)}` + row `[1 hello]` for
`SELECT 1 AS one, 'hello' AS greet` over the wire. An automated
shell harness will land in Phase 7 with the psql integration
tests.
Also rolls go.mod / go.sum forward with the pgx v5 toolchain pulled
in by Phase 2's pgproto3 dependency. Module bump 1.21.13 → 1.25.0
matches what `go get github.com/jackc/pgx/v5/pgproto3` selected;
cross-builds for windows/linux/darwin all still succeed (verified
locally).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
