Add workspace roadmap plumbing for snapshots, checkpoints, and filesystem layers

[zac]

Summary

• Add the core workspace roadmap implementation across snapshots, revisions, checkpoints, diffs, merges, and mutation tracking.
• Expand filesystem support with in-memory, overlay, sandbox, mountable, limited, and security-scoped variants.
• Add broader test coverage for persistence, compaction, transactions, patching, tracking policy, and workspace APIs.

Testing

• Not run (not requested)