feat: implement name_converter module parameter for uid/gid mapping

[oferchen]

Summary

• Implements the name converter daemon module parameter, allowing uid/gid name resolution via an external subprocess instead of NSS lookups
• Adds NameConverterCallbacks trait and thread-local converter slot to metadata::id_lookup for dependency-inverted name resolution
• Spawns a long-lived subprocess communicating via stdin/stdout line protocol (uid|gid|usr|grp {arg}\n → response line)
• RAII guard (NameConverterGuard) ensures cleanup on drop, preventing stale converters across transfers
• Wired into process_approved_module() after privilege reduction, with %-variable expansion for the command string

Test plan

• Unit tests for all four query directions (uid→name, gid→name, name→uid, name→gid)
• Broken pipe and empty response handling tests
• Thread-local interception test verifying converter overrides NSS lookups
• Guard drop test confirming converter cleanup
• Protocol dispatch test with realistic shell script converter
• Non-numeric response handling for name→id queries
• Clippy clean with all features