Feat: Add support for computing account and storage proofs with overlay state

## Description

The current `StorageEngine` implementation supports computing proofs for committed state (on-disk). However, for integration with execution layers (like Reth) that need to query pending state or simulate transactions, we need the ability to generate Merkle proofs for state that includes in-memory overlays (uncommitted changes), without persisting them to disk first.

Start by tracing the StorageEngine path that computes proofs for committed, on-disk state, then inspect how in-memory overlays are represented and queried. Compare the requirements from execution-layer integrations such as Reth. Done means account and storage proofs include pending overlay changes without persisting them, while committed-state proof behavior remains intact.

Written by the indexing model from the issue text.