upgrade consensus contracts to solidity 0.8.30

[Andrew-Pohl]

Summary

Upgrades all consensus contracts (Consensus, BlockReward, Voting, ProxyStorage, EternalStorage/Proxy + abstracts, interfaces and test mocks) from Solidity 0.4.24 to 0.8.30.

These are drop-in implementation upgrades: storage layout and all keccak storage keys are unchanged, so the new implementations can be rolled out to the live proxies through the existing ballot → ProxyStorage.setContractAddress → upgradeTo governance flow. No re-initialization is required (isInitialized remains set in proxy storage).

Compiler / EVM target

• solc 0.8.30, optimizer unchanged (enabled, 200 runs)
• evmVersion is explicitly pinned to berlin. Per config/spec.json, Fuse/Spark have Constantinople + Istanbul/Berlin EIPs activated but not Shanghai, so default 0.8.20+ codegen would emit PUSH0 and produce bytecode that cannot run on chain.

Contract changes

• SafeMath/Math removed — openzeppelin-solidity dependency dropped entirely. .add/.sub/.mul/.div/.mod replaced with native operators (0.8 checked arithmetic has the same revert-on-overflow semantics). Math.min/max replaced with internal _min/_max helpers in ConsensusUtils.
• Fallback split (behaviour preserved):
  • Consensus: receive() + fallback() both stake msg.value for msg.sender, matching the old catch-all fallback (plain send = stake, unknown selector = stake).
  • EternalStorageProxy: receive() + fallback() both delegatecall to the implementation via a shared _delegateToImplementation().
• Mechanical 0.8 migration: explicit memory/calldata data locations, arr.length-- → arr.pop(), payable(...) casts for .transfer() and proxy address conversions, abstract/virtual/override wiring (including the onlyConsensus modifier override in VotingMock), 0.8 assembly syntax (gas(), calldatasize(), ...), SPDX identifiers.
• Removed two tautological require(x >= 0) checks on uints in BlockReward (always true; compiler warnings only).

Behaviour notes

• DECIMALS - validatorFee() in delegator reward distribution was an unchecked subtraction in 0.4; it now reverts on underflow. Unreachable in practice (fee capped at 100%), but a strictly safer failure mode.
• Arithmetic overflow/underflow now reverts with Panic(0x11) instead of a silent wrap / plain revert. All existing guards (explicit requires) still fire first on the normal paths.

Tooling

• hardhat pinned to ~2.22.19 (last 2.x line fully supporting Node 18; newer requires Node 20+)
• Removed unused solc@0.4.24 devDependency (flatten uses hardhat flatten)
• yarn.lock regenerated

Testing

• yarn compile — 20 files compile cleanly (evm target: berlin)
• yarn test — 173 passing, 0 failing, including the upgradeTo suites that exercise upgrading proxies in place and verifying state survives

[mul53]

do we need to add payable here?

[Andrew-Pohl]

Pre 0.5.0 all addresses were payable. in 0.5.0 it split to non payable to payable.