docs: add cross-SDK parity guidance for code review

REVIEW.md

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+# Code review guidelines
+
+Repo-specific guidance for automated PR reviews.
+
+## Cross-SDK parity
+
+LanceDB exposes the same core (`rust/lancedb`) through Python, TypeScript (`nodejs`),
+and Java bindings. Behavioral drift between SDKs is a recurring problem, so watch for
+parity gaps when reviewing — but only flag real ones:
+
+* If the change adds or modifies user-facing API or behavior in the shared core
+  (`rust/lancedb`), check whether each binding that should expose it (`python`,
+  `nodejs`) does. A core change with no corresponding binding update is worth a note.
+* If the change adds or modifies a public API in one SDK but not the other, open the
+  sibling SDK's corresponding module and state whether an equivalent exists. If not,
+  note it as a possible parity gap and suggest a follow-up issue.
+* For bug fixes, first read the sibling SDK's analogous code path to check whether the
+  same bug exists there. Only raise parity if it actually does. Do not ask to "port" a
+  fix for a bug that only ever existed in one binding.
+* Stay silent on internal-only refactors, tests, docs, and changes with no cross-SDK
+  surface.
+* Parity expectations apply to the Python and TypeScript (`nodejs`) SDKs. Java currently
+  implements only the remote table, not the local/embedded backend, so it is expected to
+  be partial — do not flag Java for missing local-only functionality.
+* Keep parity feedback to a short, clearly-labeled note (e.g. "Possible SDK parity
+  gap: …"). It is advisory, not a merge blocker.