Method persist_registry_snapshot

Re-snapshot the materialized working registry to `registry.bin`. `Session::load` trusts the stored registry to match the persisted `head`, so any cursor move (undo/redo) that rewinds the working registry without retiring must re-persist it or a reopen would read a registry inconsistent with `head`. Synchronous and hot-path-safe (`write_non_blocking`).

(&mut self)