Add `write()` export

See #268. Add include/mechanism_configuration/exporter.hpp.

Acceptance criteria

• void write(const Mechanism&, const std::filesystem::path&) compiles
• .yaml produces YAML; .json produces JSON; unknown suffix is an error
• Round-trip: parse(path) then write(out) then parse(out) produces equal Mechanism

Ideas

• Unknown suffix must error; silent wrong-format output is worse than a clear error
• might have to do some tricks to emit json, but seems possible?
  • stackoverflow answer
  • or could always add nlohmann::json as a dependency for writing out

Read #268 and inspect the existing parse(path) behavior before starting in include/mechanism_configuration/exporter.hpp. Implement the requested write(const Mechanism&, const std::filesystem::path&) interface, then verify YAML and JSON output, unknown-suffix errors, and the parse/write/parse round-trip acceptance criteria.