Share immutable registered input bundles across research trials

[PureSaber]

Summary

• Add named immutable input bundles over the existing content-addressed artifact store.
• Publish members, dependency edges and owner pins atomically; repeated content is stored once across trials and retries.
• Verify hashes, sizes and registered dependencies on load/read; expose read-only streams, without hard links or writable CAS paths.
• Document conservative retention and limitations (serialized readers and integrity I/O; no automatic Notebook migration).

Validation

• New and affected CAS/snapshot tests: 66 passed, 6 existing platform-specific skips.
• Final focused bundle/previous full-suite dependency repair check: 14 passed.
• Final full repository regression: 1018 passed, 7 existing platform/runtime skips.
• Ruff check and format check passed across src and tests.
• Tests cover real competing publishers, killed publication/restart, tampered members, immutable revision identities, and transitive retention.

Integration

Used by quant-pipeline durable research batches. Existing SnapshotStore and ArtifactStore APIs are unchanged. No service deployment or live data collection.