Organize public runner tests by responsibility

[stef-k]

Summary

• move the 47 existing public-runner tests into six responsibility-named modules
• preserve every class, scenario, assertion, and test identity exactly once
• make each moved module work with package-qualified unittest invocation

Validation

• six focused modules: 47 tests passed
• full suite: 298 tests passed
• exact identity and AST preservation verified
• changed-file Code Guard: PASS
• git diff check: PASS

Closes #104