Checkpoint: Skip-List Traversal with Fat checkpoints

[julio4]

See #91

Modify the Checkpoint architecture to support non-linear history traversal. Currently, DatabaseRef implementations must traverse the entire linked list to find a state key. We need to introduce a "skip-list" mechanism where checkpoints can reference a "Fat" ancestor containing accumulated state.

Update CheckpointInner to include:

• accumulated_state: Option<BundleState>: The merged state up to this point.
• fat_ancestor: Option<Arc<Self>>: A direct reference to the nearest "Fat" checkpoint.

Update Traversal Logic in DatabaseRef implementation for Checkpoint to use fat_ancestor

• Check local state -> Check accumulated state -> Jump to fat_ancestor or Fallback to linear prev scan/Base state.

Explicit Accumulation

• Add a public .fat() method to Checkpoint that forces the creation of an accumulated checkpoint at the current tip.
• Use BundleState::extend to merge state transitions correctly.

CheckpointInner struct should be updated without breaking existing public API.