Add keeper crate to coordinate accountsdb and ledger

[bmuddha]

Goal

Add a keeper crate that owns the consistency boundary between accountsdb and ledger, and exposes one API for the transaction processor and SVM runtime.

Why

accountsdb stores account state. ledger tracks transaction and execution results along with block/slot context. Those two views must stay synchronized so runtime code does not duplicate consistency logic.

Scope

• Define the keeper crate and its API boundary.
• Define the coordination path that pairs account-state changes in accountsdb with execution/result recording in ledger.
• Keep accountsdb and ledger invariants aligned after each transaction or slot.
• Make the API usable from both the transaction processor and the SVM runtime.

Constraints

• Start with the smallest correct surface.
• Avoid validator-style flow and keep the design execution-engine oriented.