Allow Rustic MCP startup outside game directories

[JamieW105]

The globally registered Rustic MCP command has no --project argument. When a desktop client starts it outside a game directory, the backend exits with code 2 before responding to initialize, so the client cannot discover its tools.

Allow initialization, tool discovery, and ping without a selected game. workspace_info now accepts an absolute project_root to bind a game for the connection; subsequent project tools use that selection. Invalid replacement paths return tool errors and retain the prior game. Automatic discovery from a game directory or subdirectory still works. Project selection remains required for content tools.

Local stdio registration does not expose a user's Rustic installation to a cloud Codex chat. The companion documentation PR explains that distinction and setup/troubleshooting for the updated backend.

Validation: reproduced the old exit from the engine checkout; the updated binary responds to initialization from the same directory. All 7 backend unit/stdio regression tests pass; targeted Clippy with warnings denied and workspace formatting pass. An official MCP SDK client also successfully initializes, lists tools, receives an actionable missing-project error, and pings from outside a game. Website lint, build, TypeScript, and dependency audit pass.

Windows installer packaging passed on Windows via this workflow, producing a fresh RusticGameEngine-Setup-0.1.0-x64-20261012-012313.exe. The artifact is available as RusticGameEngine-Windows-Installer on that run. Full Linux/Windows PR checks are tracked separately by CI.