[codex] add flavor boundary check

[PerishCode]

Summary

• add flavor.json for the current Rust source and test boundary
• remove in-source test hooks flagged by flavor
• migrate behavior coverage to CLI integration tests with shorter test names
• shorten RuntimeConfig constructor name

Validation

• flavor check --root .
• flavor check --root . --strict-warnings
• cargo fmt --check
• cargo test
• cargo build