feat: make gas limit configurable from genesis (#132)

[0xMuang]

Summary

Block gas limit is now read from genesis configuration instead of being hardcoded to 30,000,000.

Changes

node.rs

1. Added DEFAULT_GAS_LIMIT constant (30,000,000)
2. Added gas_limit: u64 field to Node struct
3. In bootstrap_validators_from_genesis(): store genesis.gas_limit converted to u64
4. Updated create_genesis_block(timestamp, gas_limit) to take gas_limit parameter
5. Updated execution_result_to_block(block_number, result, gas_limit) to take gas_limit parameter
6. Updated run_event_loop() to pass gas_limit to block creation

How it works

Genesis JSON → bootstrap_validators_from_genesis() → self.gas_limit
                                                          ↓
                        create_genesis_block(timestamp, gas_limit)
                                                          ↓
                        execution_result_to_block(num, result, gas_limit)

Test Plan

• cargo check passes

Closes #132