After the chat EventSource fires 'error', attachLiveStream made a single 1.5s
reconnect probe and, if /api/chat/stream/status did not yet report active or
replay_available, fell straight through to _handleStreamError(): clearing the
owner INFLIGHT state, nulling S.activeStreamId, pushing a 'Connection
interrupted' message and re-rendering. That wiped the live message DOM even
when the backend was still producing tokens or the run-journal replay file was
a beat away from becoming visible, so the settled response disappeared and then
reappeared from sidecar/replay (or only on refresh).
Replace the single probe with a short staged retry window
(_retryDelays=[1500,3000,5000,8000] ms) driven by a recursive _probeReconnect.
Each stage re-queries stream status and reconnects/replays when the backend is
reachable; the live DOM and S.activeStreamId/INFLIGHT state are kept intact
across the whole window, and _handleStreamError is only reached after every
stage has failed.
Adds tests/test_sse_error_multi_probe_reconnect.py source-locking the staged
shape and the ordering invariant (terminal error only after the retry window is
exhausted; no inflight/activeStreamId clear mid-window).
