Add hyperEVM network, switch monad to registerCustom

[galekseev]

Adds hyperEVM (chain id 999) to Networks and changes monad (chain id 143) from register to registerCustom, so both go through the Etherscan v2 multichain API with explicit explorer URLs (hyperevmscan.com, monadscan.com) and the cancun EVM version. Version bumped to 6.9.12.

These are the two commits that were accidentally pushed straight to master; they have been rebased onto the pre-push base so they can be reviewed here. #268 reverts them from master and should be merged first.

Static Code Analysis (readability, compactness):

Both networks now use the same registerCustom shape as the neighbouring cronos entries, so the block is more uniform than before. monad moves out of the plain register group.

Dynamic Code Analysis (external APIs, interaction flows):

Both entries depend on HYPEREVM_RPC_URL / MONAD_RPC_URL env vars and the shared Etherscan API key. Verification for monad now points at api.etherscan.io/v2/api?chainid=143 rather than the named monad Etherscan preset — worth confirming that key has v2 access for both chains before relying on verify.

Efficiency (gas costs, computational complexity, memory requirements):

Not applicable — hardhat network config only, no Solidity changes.

Opinion, trade-offs and other thoughts (optional):

The monad change is a behaviour change bundled with a new-network addition; it was reviewed together here because both arrived in the same commit.

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