Add History type and merge-commit model to graph-storage

[TrueDoctor]

No description provided.

[gemini-code-assist]

Code Review

This pull request refactors the history tracking mechanism in the CRDT graph storage by introducing a dedicated History struct to manage the append-only DAG of committed deltas in topological order. It updates Rev to wrap NonZeroU128 for niche optimization, changes Delta to reference a single primary parent (with an optional Merge variant for divergent branches), and updates Session and Document accordingly. Feedback on the changes suggests avoiding the unstable let_chains feature in session.rs to maintain stable Rust compatibility, and using push instead of merge for single deltas to prevent redundant topological sorting overhead.

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[cubic-dev-ai]

1 issue found and verified against the latest diff

Confidence score: 3/5

• In document/graph-storage/src/document.rs, the history traversal used by restoration ignores secondary parents from merges, so valid deltas may be skipped and merged documents can fail apply/restore with not-in-history or missing-target errors; include merge secondary parents in traversal (and add a merge-history regression test) before merging.

<sub>Reply with feedback, questions, or to request a fix.

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[cubic-dev-ai]

No issues found across 7 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}