Route verified channel events

Implement route_events(events, identities). Trusted identities maps (tenant, channel, external_user) to an internal actor. Events have tenant, channel, external_user, conversation, event_id, integer at, and text. Sort by (at,event_id) stably. Reject unmapped identities by adding their event id to rejected. For mapped events, deduplicate (tenant,channel,event_id), then append text to a session keyed by (tenant,channel,conversation,actor). Return sessions and rejected. Input events satisfy their schema and repeated event identifiers refer to the same logical delivery. This lab assumes prior transport authentication; it does not authenticate users.