Implement portable aggregate persistence and definition migration

[fruwe]

Closes #67

Summary

• implement canonical portable aggregate serialization, restoration, package attachment seeding, strict JSON/JCS hashing, typed values, and trusted artifact resolution
• implement exact descriptor-route migration, identity-preserving runtime transforms, deterministic resource limits/audits, and atomic migrate-and-dispatch results
• extend the harness for core cases 94-115 and all six persistence profiles while preserving existing format-1 behavior and public APIs

Compatibility

• package and VERSION remain 0.0.7
• existing create/dispatch behavior is unchanged
• the CI and local harness pin the authoritative, unreleased specification and conformance commits directly; no release tag is invented

Validation

• ruff check .
• mypy src/determa
• pytest -q (178 passed)
• pinned full conformance (149 passed)
• schema synchronization and Draft 2020-12 meta-validation
• package build: wheel and sdist, including all four schemas
• git diff --check