perf: stack-allocate PlainKey encoding in hot EVM state read path

[KENILSHAHH]

Summary

Stack-allocate PlainKey encoding in hot EVM state read path

• Add PlainKey::encode_account() and PlainKey::encode_storage() returning
  fixed-size stack arrays ([u8; 20] and [u8; 52]) instead of heap-allocated Vec<u8>
• Update the four hot-path callers in database.rs (basic_ref, storage_ref,
  bucket_id_for_account, bucket_id_for_slot) to use the new stack-allocating methods
• Eliminate a heap allocation on every EVM state read during block execution

Account and storage key sizes are fixed by the Ethereum specification (20 and 52 bytes
respectively), making stack arrays the correct choice. The existing encode() -> Vec<u8>
is retained for the executor.rs use-site where an owned allocation is still required.