Deduplicate CLI parsing between con and send_rs232

[jeonghanlee]

Summary

The CLI parsing skeleton and baud-rate parsing are duplicated between con (main) and send_rs232 (main). Two independent programs.

Location

con.cpp:413-567 / send_rs232.cpp:51-140.

Premise / fate

Keep-separate (independent tools) is the likely fate; Generalize-into-shared is possible. Owner decision.

Verification

• both binaries' argument-parsing behavior unchanged if touched

Backlog (deferred). Register: docs/milestone.md. Tests land in the release cycle that schedules this.