POST /actors/<id>/inbox stays the 4a 202 'accepted' stub through
all of 4a-4d. The real inbound pipeline (peer sig verify + inbox-
bucket append + projection broadcast) is Step 5's whole topic, so
4e is closed as a deliberate scope boundary — no code change.
Step 4 fully closed (4a per-actor sub-paths, 4b token map,
4c route/3 + kernel access, 4d outbox listing + pagination, 4e
inbox-stays-stub).