feat(metering): cache flashblock trie nodes for faster state root calculation

[niran]

Summary

Cache trie nodes computed during flashblock state root calculations and reuse them across bundle metering simulations. This optimization ensures that each bundle simulation only pays the I/O cost for its own state changes, not for all previous flashblock state changes.

Problem

Without caching, every bundle simulation must:

1. Compute the hashed post-state for all flashblock changes
2. Read trie nodes from disk for all accounts/storage touched by flashblocks
3. Then compute the bundle's incremental changes on top

With many bundles being simulated per flashblock, this redundant I/O becomes a bottleneck.

Solution

Introduce FlashblockTrieCache that:

1. Caches TrieUpdates and HashedPostState from flashblock state root computation
2. Uses arc_swap::ArcSwap for lock-free reads and atomic updates
3. Prepends cached data to bundle's TrieInput via prepend_cached()

Changes

• flashblock_trie_cache.rs: New module implementing FlashblockTrieCache with store() and load() methods.
• meter.rs: Add FlashblockTrieData struct and cached_flashblock_trie parameter to meter_bundle. Use TrieInput::prepend_cached() when cached data is available.
• rpc.rs: Integrate with FlashblockTrieCache to load cached trie data for bundle simulations.
• lib.rs: Export new types.
• Cargo.toml: Add arc-swap and reth-trie-common dependencies.

Performance Impact

Before: Each bundle simulation reads trie nodes for all flashblock changes
After: Each bundle simulation only reads trie nodes for its own changes

The improvement scales with:

• Number of accounts/storage slots modified by flashblocks
• Number of bundles simulated per flashblock

Test Plan

• Existing metering tests pass
• State root calculation produces correct results with cached data
• cargo clippy passes