perf: cache selfbalance and prevrandao values

[rakita]

Summary

• Cache selfbalance in Interpreter via Option<Box<U256>> to avoid repeated host balance lookups within the same frame. Cleared on clear() (frame init).
• Cache prevrandao in LocalContext via Option<Box<U256>> since it is constant per block. Added prevrandao() method to LocalContextTr trait. Changed Host::prevrandao from &self to &mut self to allow caching.

Test plan

• cargo build passes
• cargo build -p op-revm passes
• cargo clippy clean