Spec: Define sanitizer strategy for vix check

Context

Now that vix run supports sanitizers in script mode, we need to define whether and how
vix check should expose sanitizer-related analysis.

Open questions

• Should vix check support:
  • UBSan-only analysis?
  • Compile-time UB detection?
  • Conversion warnings (-Wconversion, -Wsign-conversion, etc.)?
• What should be the default sanitizer behavior?
• How should exit codes be mapped?
• Should sanitizer output be structured or raw by default?

Scope

Specification only — no implementation in this issue.

Acceptance criteria

• Clear list of supported sanitizer modes for vix check
• Defined flags and behaviors
• Exit code strategy documented