refactor(modbus): switch to native Modbus TCP protocol

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

Cargo.lock

@@ -2870,7 +2870,6 @@ dependencies = [
  "futures-core",
  "futures-util",
  "log",
- "smallvec",
  "thiserror",
  "tokio",
  "tokio-util",