Refactor invariants (maintainer)

Refactors are allowed to change structure, but not the contract.

Must stay true

• trace schema versioning remains meaningful (no silent breaking changes)
• failure taxonomy remains total (profiles cover all classes)
• canonical pipeline definition remains stable unless explicitly versioned
• docs checksum invariant continues to pass

If you must break something

• state the break explicitly
• bump the relevant version (trace schema and/or contract)
• update spec pages and checksums