Explicit process document targets

Upgrade required

• ProcessStartBindingDescriptor requires documentKey instead of definitionKey. Update named arguments and serialized catalogs; the old field is rejected. Pair it with processId to select the executable process.
• Host test fixtures create the document through createBpmnDocument() before calling createProcessDefinition() with bpmn_document_id and bpmn_process_id.
• Requires the matching Core 0.8.0 implementation and updated consuming Apps. Rebuild isolated App artifacts and reinstall their catalogs before activation.