fix: use integer arithmetic for base fee calculation (#133)

[0xMuang]

Summary

Replace floating point arithmetic with EIP-1559 integer math in base fee estimation to prevent precision loss with large values.

Changes

eth/api.rs (Issue #133)

Before (floating point - precision loss possible):

let increase = (last_base_fee as f64 * 0.125 * (last_ratio - 0.5) * 2.0) as u128;

After (integer - EIP-1559 standard):

let gas_target = last_gas_limit / 2;
let delta = (last_gas_used - gas_target) as u128;
let change = last_base_fee * delta / gas_target as u128 / 8;

This follows the EIP-1559 specification:

• gas_target = gas_limit / 2
• base_fee_change = parent_base_fee × |gas_used - gas_target| / gas_target / BASE_FEE_MAX_CHANGE_DENOMINATOR
• BASE_FEE_MAX_CHANGE_DENOMINATOR = 8

Test Plan

• cargo check passes
• cargo fmt --check passes

Closes #133