Add FilesystemStoreV2 with paginated listing support

Implements PaginatedKVStore traits with timestamp-prefixed filenames for
newest-first pagination and [empty] directory markers for consistent
namespace hierarchy. Includes v1 to v2 migration utility.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: benthecarman

lightning-persister/src/fs_store_common.rs

@@ -1,6 +1,7 @@
-//! Common utilities shared between [`FilesystemStore`].
+//! Common utilities shared between [`FilesystemStore`] and [`FilesystemStoreV2`].
 //!
 //! [`FilesystemStore`]: crate::fs_store::FilesystemStore
+//! [`FilesystemStoreV2`]: crate::fs_store_v2::FilesystemStoreV2
 
 use crate::utils::{check_namespace_key_validity, is_valid_kvstore_str};