feat: lazy fetching

Currently, Syncer maintains a continuous range of headers from Tail to Head to be stored locally. If Tail is not the genesis header, users can't request headers below Tail height. This limitation is problematic, forcing users to configure the correct SyncFromHash/Height with node restarts, and requiring them to store headers locally, while there are use cases where you only want to load them without storing.

Start by reading the Syncer implementation and tracing how it maintains the local Tail-to-Head header range. Determine how requests below Tail and loading headers without storing them should work; done means users no longer need restarts or local storage for those use cases.

Written by the indexing model from the issue text.