Separate Before Serializing Shared State

1. Identify shared mutable state (files both read and write, branches both push to, APIs both define and consume).
2. Default: eliminate the shared write target. Ask: do these actors need one canonical object, or are they publishing independent facts? Give each actor its own owned file, key, branch, or state directory, and merge only at the read/reporting boundary. Two workers writing their own

   lastX

   field into one

   state.json

   is still shared mutation;

   indexer-state.json

   +

   metrics-state.json

   is not.
3. Only when one shared write target is a real invariant, serialize access structurally (lockfiles, sequential phases, single-writer actor, or atomic compare-and-swap). Treat "we need a lock" as a design smell to check, not as the default answer.