start

Starts the inbound event loop and the GossipView. Idempotent only if called once per scope; call exactly once.

The loop is the sole collector of base.incoming (ADR-034), merged with a periodic reorder-grace sweep tick so all dedup/reorder mutation and all spool delivery happen on one coroutine — no second mutator, no lock, and released frames can never interleave out of order. When the base seam tears its incoming completes, the loop ends (cancelling the sweep ticker), and spool is closed so this seam's own incoming completes too — propagating Torn to our consumers.