Implement Mempool Backpressure

Implement backpressure mechanisms that reject new transaction submissions when the mempool reaches capacity limits to prevent unbounded memory growth and system instability. Define appropriate mempool size limits based on available memory and expected transaction processing rates. When the mempool is full, reject new transactions with clear error messages indicating the system is overloaded and clients should retry later. Implement fair eviction policies if needed, such as removing the oldest transactions or lowest priority transactions when space is needed. Expose mempool metrics including current size, capacity, and rejection rate to enable monitoring and capacity planning. Provide configuration options for mempool size limits and backpressure thresholds. This ensures the system remains stable and responsive even under extreme load conditions.

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**Quarter:** Q3

Start by locating the mempool implementation and transaction submission entry point, then trace how capacity and transaction processing are currently handled. Done means bounded memory use, configurable limits and thresholds, clear overload rejections, any documented eviction behavior, and metrics for size, capacity, and rejection rate.

Written by the indexing model from the issue text.