Function mergeField

Per-field three-way merge over (local, remote, snapshot).

• No snapshot (first sync of this id): take remote.
• Local unchanged, remote unchanged → no-op (return local).
• Local unchanged, remote changed → take remote (pull).
• Local changed, remote unchanged → keep local (push will write it).
• Both changed → remote wins (the integration's source-of-truth policy).

Equality is === for primitives (numbers, strings, booleans). For object / array values use mergeDeep instead.