Fix fast-path JSON-RPC thread contract

[Daliys]

Summary

• cache Unity version with the other main-thread editor state
• make fast-path status/wait calls avoid direct Unity API and Unity console logging on the listener thread
• document the fast-path contract and add background-thread coverage

Validation

• PYTHONDONTWRITEBYTECODE=1 scripts/prepush-validate.sh --static-only
• git push pre-push validation, including quick live server smoke
• Unity asset refresh/import idle with no compiler errors
• raw get_server_status listener-thread smoke

Fixes #79