Pre-factor: MethodRegistry generic utility

[harrryyd]

What to build

A thin, reusable MethodRegistry<Key, Value> utility module in packages/contracts/src/. Behaves like a Map but validates at registration time — rejects duplicate keys with a clear error. Accepts register(key, value) and exposes get(key) / has(key) / entries().

This is the shared infrastructure that both the RPC method registry (Candidate 1) and the command handler registry (Candidate 2) will use.

Acceptance criteria

• MethodRegistry supports register(key, value) — throws on duplicate key
• MethodRegistry supports get(key) → Value | undefined
• MethodRegistry supports has(key) → boolean
• MethodRegistry supports entries() → Iterable<[Key, Value]>
• Published from packages/contracts/src/ alongside existing exports
• vp check and vp run typecheck pass

Blocked by

None — can start immediately.