Define and implement mempool support

## Why
Mempool support has been requested in exchange contexts, but it has not been a top priority so far.

## What
Design and implement baseline Rosetta mempool support for exchange use:
- `/mempool`: return transaction identifiers currently pending in the node mempool view.
- `/mempool/transaction`: return pending transaction details in Rosetta transaction shape, with documented field availability for unconfirmed data.
- Define source-of-truth and consistency model between node and indexer for pending data (including freshness expectations).
- Define behavior contract and error semantics for dropped/replaced/not-found transactions, short mempool visibility windows, and lag.
- Execute an acceptance validation plan covering contract correctness, edge cases, and operational reliability under realistic pending-transaction conditions.