Generate contract types, not just scope names #299

Part of #278

gen_contract.py already generates scope names and the contract
version into _contract.py with a CI drift guard. frames.py and types.py
still hand-mirror about 308 lines of shared's field-level types, which is a drift
class waiting to happen and would triple with Java.

Acceptance criteria

Python payload types are generated from one schema. The
drift guard covers fields, not only scope names. frames.py shrinks to builders
over generated types.

Size: M. Depends on B1.