fix(txpool): sort transactions by effective tip instead of max_fee_per_gas

[wpank]

Summary

Fixes a critical EIP-1559 ordering bug where the transaction pool sorted by max_fee_per_gas instead of the effective tip (priority fee). This caused transactions with high max_fee but low priority_fee to be incorrectly prioritized over transactions with genuinely high tips.

Per EIP-1559:

effective_gas_price = min(max_fee_per_gas, base_fee + max_priority_fee_per_gas)
effective_tip = effective_gas_price - base_fee

Impact of the bug:

• Validators lost fee revenue (low-tip txs were included first)
• Users paying high priority fees saw no benefit
• Pool eviction dropped high-tip txs in favor of low-tip/high-max-fee txs
• DEX/MEV ordering was incorrect

Changes

• ordering.rs: Add base_fee field to OrderedTransaction. Update Ord to sort by effective_tip() instead of effective_gas_price. Update replacement logic in SenderQueue to compare tips.
• validator.rs: Fix effective_gas_price() to compute min(max_fee, base_fee + priority_fee) for EIP-1559/4844/7702 txs. Add max_fee_per_gas() helper for balance checks. Add with_base_fee() builder to TransactionValidator. Fix min gas price check to use max_fee_per_gas (not effective price). Fix max_tx_cost() to use worst-case max_fee_per_gas.
• pool.rs: Add base_fee tracking (set_base_fee()/base_fee()) to TransactionPool. Fix tx_to_ordered() to compute correct EIP-1559 effective gas price. Update eviction to use effective_tip().
• rpc/txpool.rs: Update test helper for new OrderedTransaction::new() signature.

Test plan

• All 86 existing txpool tests pass
• New eip1559_ordering_by_effective_tip test verifies correct ordering: a tx with max_fee=50, priority_fee=50 is ordered before a tx with max_fee=100, priority_fee=1 (at base_fee=10)
• effective_gas_price_eip1559 test updated to verify correct computation at multiple base fee levels
• cargo check passes for kora-txpool, kora-rpc, and kora-runner
• Run full e2e tests

Closes #7

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