B-1: Add GetGadgetState IPC for /api/gadget_state

[mphacker]

Phase 5.28 /api/gadget_state returns 503 not_implemented because services/teslafat_client.py has no IPC method to query the Rust gadget worker's state.

Need to:

1. Add a GetGadgetState request/response to crates/teslausb-core/src/ipc/messages.rs (versioned envelope).
2. Implement the handler in rust/crates/teslausb-worker.
3. Add teslafat_client.get_gadget_state() Python wrapper.
4. Update api.py route to delegate to it (currently returns 503).
5. Update test fixtures.