docs: add managed cloud roadmap

[Davidobot]

Summary

• Add docs/cloud-roadmap.md: a sequenced roadmap for an optional managed cloud companion (the DuckDB/MotherDuck model), grounded in LodeDB's current architecture.
• Reuses the generation-addressed commit format as the cloud durable-storage substrate; no new on-disk format.
• Preserves the local invariants (loopback-only bind, payload boundary, single-writer crash-atomic commits, lean core); all cloud code stays behind a separate package and optional extra.
• Hybrid local/cloud is retrieval routing and result fusion, not relational operator splitting (LodeDB has no SQL optimizer).
• Marks Milestones 0-2 (self-hosted/BYOC durable sync) as the near-term low-risk slice and the control plane onward as demand-gated.
• Links the roadmap from the README enterprise note and the architecture overview.

Testing

Documentation only; no code or dependency changes. The architectural claims it relies on (generation-addressed commit format, atomic root-manifest swap, loopback bind guard, read-only handles, payload sidecars, import boundary) were checked against the current source.