Protect stored events from caller mutation

[EauDoon]

Summary

• Clone event payloads before signing and freezing them in the in-memory store.
• Return independent snapshots from append and list operations.
• Prevent caller mutation from freezing inputs or altering the stored signed event chain.

Verification

• Repository check and tests: 131 passed.
• Focused rail suite: 77 passed.
• Stored snapshots remained stable after input and returned-value mutation.
• Exact-tree and full-history scans introduced no new finding.

The documented loopback-only experimental boundary remains unchanged.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.