Harden RedXaiHM transactions, mutations, locks, and recovery

[RedXDevelopment]

Summary

Strengthens the RedXaiHM alpha before any production identity migration.

• Makes every direct SDK mutation atomic: validation failures restore the exact last valid document and rebuild indexes.
• Remaps ID references that point inside copied subtrees while leaving references to outside nodes unchanged.
• Holds the file lock across the entire load/mutate/save transaction, preventing lost updates from concurrent writers.
• Adds an active lock heartbeat so long-running valid transactions are not stolen as stale.
• Uses random temporary names, fsyncs atomic backup replacements, and cleans failed temporary writes.
• Adds concurrency, stale-lock, corruption, permissions, rollback, reference, and copy-integrity tests.

The branch does not add encryption; that remains the next dedicated milestone after mail registration and API end-to-end verification.