fix(store): publish lock ownership atomically

[trvon]

Summary

• publish complete lock ownership atomically
• prevent empty lock files from being stolen during initialization
• add deterministic persistence race coverage

Validation

• focused persistence suite: 110 tests passed
• full branch suite: 905 tests passed

Stack

1 of 6. Merge before the execution-identity authority PR.

[Copilot]

🟡 Changes recommended

The new stale-lock cleanup can delete malformed-but-non-empty lock contents, which can allow a live lock to be stolen under partial/truncated content scenarios.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR hardens ReducerBackedStore’s file lock protocol so that lock ownership is published atomically (preventing a second writer from “stealing” an empty lock inode during initialization), and adds a deterministic regression test to cover the persistence race.

Changes:

• Reworks lock acquisition to write an owner record to a candidate file and atomically “publish” it via linkSync, and updates unlock to verify ownership before removing the lock.
• Updates lock parsing to accept both legacy pid and new pid:uuid formats.
• Adds a deterministic test (AUD-10) ensuring an empty live lock can’t be replaced or entered by a second writer.

File summaries

File	Description
src/reducer-backed-store.ts	Implements atomic lock-owner publication via hard-linking and owner-verified unlock semantics.
test/reducer-backed-store.test.ts	Adds deterministic coverage for the “empty live lock cannot be stolen” race scenario.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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