feat: add NetworkUpgradeTxns library

[0xiamflux]

Note

Introduce a Solidity library to build/serialize network upgrade transactions (NUT) and add tests, including Ecotone vectors and JSON roundtrip.

• Libraries:
  • Add packages/contracts-bedrock/src/libraries/NetworkUpgradeTxns.sol providing:
    • NetworkUpgradeTxn struct mirroring DepositTx fields.
    • newTx to construct tx metadata with intent-derived sourceHash.
    • sourceHash domain-separated hashing (domain=2).
    • JSON IO: writeArtifact, serializeTxn, readArtifact using Foundry Vm.
• Tests:
  • Add packages/contracts-bedrock/test/libraries/NetworkUpgradeTxns.t.sol with:
    • Source-hash vector checks against Ecotone intents.
    • newTx field mapping property test.
    • Artifact write/read tests (empty, single, multi-tx).
    • Full Ecotone upgrade roundtrip including proxy upgrades and EIP-4788 deployment.

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