feat(evmonly): add eth_getBalance RPC - #4140

Adds eth_getBalance to the EVM-only JSON-RPC surface, reading the current committed EVM state and rejecting historical selectors, with unit, registration, and Docker integration coverage. The block-selector gating, the nil Sei-address argument (only reachable in EVM-only mode, where it is ignored), the RPC-surface exposure, and the 2^200 + 1 integration assertion all check out; nothing blocking found.