SSE stream for a project chat conversation

Session-authenticated observation stream for the project's single shared chat conversation; the session is the project conversation, not one program's. Emits hello/turn/change_set/delivery_recovery/status/error/ping events and replays stored turns on reconnect. A scope conversation also emits scope ({ rev }) whenever the project's scope map revision moves, so the portal refetches the map; roadmap conversations never emit it. Proposal notifications carry the exact ID, revision and status; status uses running/idle/terminated derived from durable processing state and reports uncertain delivery and processor errors. Does not execute provider or tool calls; durable processing continues without viewers. Current project access is rechecked on every transcript read.