refactor(run): migrate direct script compilation to Vix Engine

[GaspardKirira]

Summary

Move direct single-source compilation planning and execution from
modules/cli into Vix Engine.

Requirements

• Move compiler selection into the engine.
• Move compiler fingerprinting into the engine.
• Move direct compilation planning into the engine.
• Reuse engine process execution.
• Reuse engine cache services.
• Return the produced executable artifact.

Acceptance criteria

• Direct compilation no longer depends on CLI styling.
• Direct compilation returns engine artifacts.
• Compiler diagnostics are structured.
• Cache decisions use engine services.
• vix run main.cpp remains compatible.