NS-21: add GdUnit4, client tests, and CI headless run
- Vendor gdUnit4 v6.1.2 under client/addons; enable plugin in project.godot
- Tests: player.gd API, position authority HTTP flow (Node transport double),
ground_pick walkable ancestry; test double script for injection
- position_authority_client: _create_http_request factory, Node-typed _http,
emit move_rejected unknown for empty/missing reasonCode strings
- gdscript workflow: lint scripts+test, format check, cache Godot 4.6, GdUnit CLI
- README testing section; gitignore client/reports; testing-expectations Godot harness
VinPropane
8113e48cc8