bench: add connection and query benchmarks for SQLite, Postgres, MySQL

[Map130]

Closes #158.

Adds criterion benchmarks covering all items from the issue checklist, for all three supported databases.

What's included

Each database gets a benches/<db>/connection.rs with five benchmarks:

Benchmark	Description
new_connection	Establish a fresh connection from scratch
pool_checkout	Acquire an idle connection from a pool (min=max=1)
ping	Ping an existing open connection
query_small_result	Fetch 1 row by PK (TechEmpower single-query style)
query_large_result	Fetch 10 000 rows with TEXT and BLOB/BYTEA columns

How to run

SQLite (no external DB needed):

cargo bench --bench sqlite-connection --features sqlite,runtime-tokio

Postgres:

DATABASE_URL=postgres://... cargo bench --bench postgres-connection --features postgres,runtime-tokio

MySQL (mysql-rsa enables caching_sha2_password auth without TLS for MySQL 8+):

DATABASE_URL=mysql://... cargo bench --bench mysql-connection --features mysql-rsa,runtime-tokio

The existing tests/docker-compose.yml can be used to spin up Postgres and MySQL.