Add dependency checks for x-mapper mapping program

[wwood]

Summary

• add a MappingProgram::check_dependencies helper so each mapper, including x-mapper, validates its external tools when selected
• update the CLI parser to invoke the shared dependency check and adjust the x-mapper command test expectation
• resolve recent clippy findings uncovered by the hook

Testing

• cargo fmt
• cargo clippy -- -D warnings
• cargo test

---

Codex Task