rfc: add RFC 0050, engine crate topology - #615
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Draft the decision record for splitting the engine package into a sealed substrate crate, a logical authority crate, an executor crate, a write-protocol crate, and a thin facade, with typed seams and the guards that make the boundaries compiler facts. Registers 0050 and advances the next available number.
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Drafts RFC 0050: split the engine package into a seven-crate family with a sealed substrate boundary. omnigraph-lance is the only production crate that links Lance and DataFusion execution. Its operational API is Lance-free, while a closed compatibility module preserves the four existing read-only SnapshotDataset and SnapshotScanner signatures that already name Lance types.
The RFC now makes the authority boundaries explicit without changing user behavior: omnigraph-graph owns accepted graph state and captured write authority; omnigraph-exec owns typed read execution; and a policy-configured omnigraph-write owns supported graph-write orchestration. Existing actor-less maintenance dispositions remain unchanged.
Graph publication is separated from physical CAS. publish_once performs graph-aware validation and bounded retry, while omnigraph-lance exposes a single-attempt manifest CAS tied to an exact physical observation. Exact-authority operations keep a fixed WriteTxn; existing explicitly rebasable adapters retain that behavior. Only proven no-commit contention retries, while ambiguous outcomes remain recovery-owned.
The facade keeps the current public Rust surface by re-export. New crates publish in dependency order, with API snapshots and structural guards covering the compatibility allow-list, policy dispositions, and the sole workspace production caller of the physical manifest CAS.
Process: draft status per step 2 of the RFC guide. Registers 0050 and moves the next available number to 0051.
Checks: python3 scripts/check-docs.py, bash scripts/check-agents-md.sh, python3 scripts/check-workflow-action-pins.py, and git diff --check pass.