Switch from Modbus RTU over TCP to native Modbus TCP based on hardware
testing. Uses standard MBAP header (no CRC16), port 502, and TCP-only
tokio-modbus feature for simpler implementation.
Updated: Cargo.toml, plan.md, research.md, tasks.md
Adds tokio-modbus 0.17.0 for Modbus RTU over TCP communication,
sqlx with runtime-tokio and sqlite features for async database operations,
mockall 0.13 for trait mocking in tests, and async-trait 0.1 for async trait support.
Ref: T001 (specs/001-modbus-relay-control)
