Spec: Standardize CLI exit codes

Goal

Define a consistent exit-code strategy across all CLI commands:

• vix run
• vix dev
• vix check
• vix tests
• vix build
• vix pack
• vix verify

Topics

• Build failure vs runtime failure
• Test failures
• Sanitizer-detected failures
• User interruption (SIGINT)

Acceptance criteria

• Documented exit-code table
• Clearly defined semantics for each code