Implement sole schema-v2 artifact and checkpoint contract

[fruwe]

Summary

• make artifact and checkpoint schema version 2 the sole supported portable artifact contract
• remove all artifact/checkpoint v1 schemas, APIs, variants, wrappers, conversion code, drivers, documentation, and compatibility tests
• expose only the queue-bearing create, admit, and step execution model; keep mailbox-free engine helpers private
• implement native v2 aggregate, package, migration, checkpoint, admission, stepping, maintenance receipt, retention, and optional durable-host behavior
• require target_validated_bundle_fingerprint and enforce migration receipt chronology, replay-before-CAS, conflicts, stale writers, audits, and migration sequence continuity
• execute all checkpoint and persistence profiles through production ExecutionHost and ExecutionStore transaction paths
• require backup manifests to include current definitions, historical identity origins, fault anchors, and migration descriptor recovery routes
• resolve every required backup definition and migration descriptor, require resolver trust, and verify each content-addressed fingerprint or digest
• validate exact closed public host responses, persisted bytes, exercised broker-adapter acknowledgements, all manifest artifacts, and backup/restore completeness without behavior selected from covers, expect, or result artifacts
• update bundled schemas, CI, documentation, and exact immutable pins

Validation

• ruff check .
• mypy src/determa
• pytest -q (248 passed, 5 local PostgreSQL skips because no DSN was configured)
• exact pinned conformance: 808 passed, covering 98 format-1 cases, all 162 version-2 vectors, all 138 durable-host vectors, all 381 manifest artifacts, and focused adversarial host and backup-artifact checks
• package build and wheel/source-distribution content verification
• GitHub: Python 3.11 compatibility, unit tests, conformance on Python 3.11 and 3.13, and PostgreSQL all passed on the current head
• schema synchronization against spec ee38796d5e38e67e350a06548fd50faa530cbb12
• conformance pinned to 99a4d9ad5256f7330e75b06d48f340cc7239a40d
• forbidden compatibility scan confirms no artifact/checkpoint v1 operation or upgrade/downgrade API remains
• git diff --check

Scope

• machine YAML remains format: 1
• package version remains 0.2.0
• durable-host profiles remain optional library contracts
• no merge, tag, publication, or release work

Closes #84