Verbatim move of LocalStateBackend, StateSnapshot, StateLockGuard and their
impls — the single home for stored-state I/O (state ledger, lock, recovery
sidecars, approval artifacts), where the RFC-006 object-storage port lands
next as a focused diff. Visibility bumps (pub(crate)) only; 95 tests green
before and after.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
