refactor: migrate from custom bitmask to EnumerableSet architecture

[rplusq]

Description

This is a follow-up PR on top of @xDarksome #212. My goal was to keep Rust/WCN integration almost unchanged while modernising the Solidity internals.

What changed (one-liner each)

• Membership: custom 256-slot bitmask → OpenZeppelin EnumerableSet.
• Ownership: custom owner fields → Ownable2Step (safer hand-off).
• NodeOperator struct: now includes maintenance boolean (replaces separate mapping).
• Migration FSM: same API, but isPulling set in a single loop (gas –5 k/op).
• On-chain maintenance toggle: setMaintenance(bool) emits MaintenanceToggled.
• Custom errors replace string reverts.
• Settings & version: every mutator bumps version for subgraph indexing.

Integration side of things

• All public getters (getCurrentKeyspace, getMigrationStatus, …) unchanged.
• Events names unchanged + new MaintenanceToggled (ignored by nodes until used).
• Gas profile: startMigration(16 ops) ≈ 140 k, setMaintenance ≈ 20 k.
• Migration-to-main-net plan (unchanged)
• Backend points to new address, runs e2e tests.

@xDarksome : please confirm events + getters still fit alloy bindings. No single getView() now, so you might have to join with multiCall on the alloy side: https://alloy.rs/guides/multicall#multicall-with-alloy

All forge tests green; ready for merge once you’re happy.