test(run): preserve existing vix run CLI compatibility

[GaspardKirira]

Summary

Create compatibility tests for supported vix run invocation forms before
removing legacy implementations.

Coverage

• project directory;
• project target name;
• existing executable;
• single C++ source;
• .vix manifest;
• runtime arguments;
• environment variables;
• working directory;
• sanitizers;
• temporary dependencies;
• watch mode;
• replay;
• check-only mode.

Acceptance criteria

• Existing successful commands remain successful.
• Existing runtime arguments retain their boundaries.
• Existing environment behavior remains supported.
• Removed --auto-deps options return a migration diagnostic.
• Exit-code behavior is tested.
• Help output reflects the final supported interface.