Plugin architecture

The plugin model has a contract-time half and a runtime half. Together they make the harness a small addition to a host instead of a parallel system.

• harnessContract() declares the harness surface and collections.
• harness(engine) supplies RLS policies, handlers, and the event-to-storage bridge.
• createHarnessClient() reads the resulting tree through the host's client.

All three use @super-harness/shared as the wire ABI, so server and client must deploy compatible versions.