[beinan]

Summary

• Add an opt-in raft-log storage boundary: one physical engine per node and cloneable raft-rs Storage handles for its static session-sharded groups. Persist entries, term ranges and HardState atomically, then wait for a shared physical sync barrier. Bind the layout and node identity before returning any handles.
• Reject committed-prefix replacement and term/vote/commit regressions. Look up terms without decoding payloads, and bound each engine range-read request before applying Raft's byte limit. A disk-write error or unwinding sync panic permanently fences every group handle until the physical log is reopened.
• Cover real files, independent-engine cache isolation, concurrent sync/error paths, Linux child-process write failures and SIGKILL recovery, and the RawNode persistence/Ready contract. Use the vendored cache correction from fix(wal): isolate raft-engine entry caches by engine #398 and run the optional storage tests in CI.

A successful local persist is not a quorum ACK. This increment retains the full log and static membership; transport, snapshots/reclamation, producer deduplication and Lance publication remain follow-up work. It does not switch production ingestion.

Refs #378.

Testing

• cargo test --locked -p lance-context-wal --features raft-log --lib --test raft_log --test raft_log_cache_identity: 18 passed; the subprocess worker is invoked by its parent tests and ignored as a standalone test.
• The three unchanged facade cache tests returned exact wrong payloads on the original engine and passed with the vendored correction.
• cargo clippy --locked -p lance-context-wal --all-features --all-targets -- -D warnings: passed.
• Full ci-pr-helper checks passed: workspace Rust tests, Python (229 passed, 2 skipped, 1 expected failure), Ruff, Pyright, formatting and Clippy. Workspace package metadata and spelling checks passed.
• The final term-decoding iteration also passes Rust 1.99 Clippy and all 18 durable-storage tests; the truncated-index check and on-disk representation are unchanged.