fix(txpool): add EIP-2 signature malleability check and unify tx hash computation

[wpank]

Summary

Fixes two critical security issues in the transaction pool:

• EIP-2 signature malleability (closes #195): The validator accepted ECDSA signatures with high s values, allowing an observer to take any valid pending transaction and produce a second valid transaction with a different hash but the same sender and nonce. This pollutes the mempool and breaks hash-based transaction identity. The fix adds a normalize_s() check after Signature::from_slice() to reject signatures where s is in the upper half of the secp256k1 curve order.

• Inconsistent tx hash computation (closes #197): The Mempool::insert() path computed the transaction hash as keccak256(raw_2718_bytes) while the validator path computed it as keccak256(rlp_encode(envelope)). Neither is the canonical Ethereum transaction hash (keccak256(encode_2718(envelope))), and the mismatch meant the same transaction inserted via different paths would have different hashes in the pool's by_hash map, bypassing duplicate detection. Both paths now use envelope.tx_hash() which returns the canonical hash.

Additionally fixes a pre-existing compile error where rejection_reason() was missing match arms for GasLimitTooHigh and InitcodeTooLarge error variants, and removes the now-unused alloy-rlp dependency.

Changes

• crates/node/txpool/src/validator.rs: Add EIP-2 low-s check, use canonical tx_hash(), remove unused keccak256 import
• crates/node/txpool/src/pool.rs: Use canonical tx_hash() in tx_to_ordered(), add missing match arms in rejection_reason()
• crates/node/txpool/Cargo.toml: Remove unused alloy-rlp dependency

Test plan

• Existing validator tests pass (they use k256::ecdsa::SigningKey::sign_prehash_recoverable which produces low-s signatures by default)
• Verify that a manually constructed high-s signature is rejected by recover_sender_and_hash()
• Verify that Mempool::insert() and validator validate() produce the same hash for the same transaction

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