test: add minimal compaction correctness gate

[belveryin]

Summary

• Add RFC docs/rfcs/0012-compaction-correctness-gate.md defining Phase 0 invariants and oracle model.
• Introduce a minimal MVCC oracle and deterministic Phase 0 scenarios: overwrite chain, delete-heavy, range scan with deletes, and cross-segment overlap.
• Ensure deterministic compaction forcing with explicit outcome assertions; stabilize flush behavior via flush_immutables_to_l0 and disabled minor compaction in the harness.

Tests

• cargo test compaction_correctness_
• cargo test

[ethe]

The new compaction correctness scenarios reuse the same TxSnapshot before and after compaction, so post-compaction reads still use the pre-compaction manifest snapshot. This won't catch bugs where compaction writes incorrect SST outputs, because the new SSTs aren't read at all.

[belveryin]

Good catch. I fixed it by taking a fresh snapshot after compaction (asserting the read_ts is unchanged) and re-running the oracle/range checks on that snapshot, so we now validate the compaction outputs.