Keystead Core 0.5.3

This release fixes vault persistence and recovery defects found during the Keystead review, and adds APIs needed for secure-note editing and explicit sync conflict resolution.

• Roll back failed compound mutations, including revisions, headers, records, tombstones, and key rotation state.
• Refuse overwriting a restore destination even when it is created concurrently; reject file symlinks that could bypass the vault lock.
• Reject malformed sync timestamps individually and support backup encoding expansion for valid large records.
• Add updateSecureNote and authenticated resolveSyncRecord; explicit resolution preserves record identity and emits a newer revision.

Local validation: 419 tests passed, with 9 platform-dependent skips; Javadoc and formatting passed. New regression tests cover persistence failures, restore races, large backups, malformed timestamps, symlinks, and explicit conflict resolution.

Maven coordinates: top.focess:keystead-core:0.5.3.

Release CI passed on Windows x86-64, macOS ARM64, Linux x86-64, and Linux ARM64. Failure-injection and large-backup test fixtures were made portable without changing production behavior or skipping their assertions.