refactor: split pipeline_state.rs into 4 sub-modules with co-located tests
The 1411-line pipeline_state.rs is split into:
- mod.rs: types, transition(), execution_transition(), labels + transition tests (885 lines)
- events.rs: TransitionFired, EventBus, TransitionSubscriber + event-bus tests (114 lines)
- projection.rs: ProjectionError, TryFrom<&PipelineItemView>, read_typed + projection tests (379 lines)
- subscribers.rs: 5 concrete TransitionSubscriber stubs (95 lines)
Tests stay co-located.
No behaviour change. All 42 pipeline_state tests pass; full suite green.
dave
5aedf94512