test(run): add cross-platform run fixtures

[GaspardKirira]

Summary

Create deterministic fixtures covering scripts, projects, binaries, processes,
watch mode, dependencies, and failures.

Fixture coverage

• simple C++ source;
• Vix runtime source;
• source requiring CMake fallback;
• project with one executable;
• project with multiple executables;
• non-zero runtime exit;
• signal termination;
• timeout;
• high-volume output;
• interactive input;
• watch rebuild and restart.

Acceptance criteria

• Linux fixtures are included.
• macOS fixtures are included.
• Windows fixtures are included.
• Direct and fallback script paths are covered.
• Runtime process behavior is covered.
• Watch behavior is covered.
• Tests avoid timing-dependent assertions where possible.