tear

Transition state to SeamState.Torn with reason, collapsing peers to { selfId } and completing incoming. Idempotent.

The spool: every real seam (NwSeam.latchTorn, MeshSeam.tearDown, InMemorySeam.close) closes the spool on tear, so a consumer that relies on incoming completing (or wrongly processes frames after Torn) can't pass against this fake but fail in production.

The roster: Seam.peers requires a Torn seam's roster to be exactly { selfId }, published before the terminal latch — hence the _peers write ahead of _state below. Read the seams named above as a model for the spool dimension only. On the roster one they now all collapse to { selfId } (#1849 fixed InMemorySeam, whose peers derives the collapse from its close latch), but they do not all publish it before the latch: NwSeam.close latches Torn and collapses afterwards, in a later lock acquisition (#2648). The obligation this tear honours comes from Seam.peers and SeamConformanceSuite.peersCollapseToSelfIdWhenTorn, not from any one of them; it is pinned here by TestFakePeersCollapseOnTearTest (#1854).