What:
- M5: `relay` is a gateway surface alias, so an inbound the connector could not stamp is
entrypoint gateway_message / surface gateway (it counts as engagement) instead of other/none;
`relay` joins the task/session gateway platform set (it was collapsing to `plugin`).
- m9: RelayAdapter._metrics_platform labels a chat by its inbound platform, and falls back to the
connector's primary platform only when the socket fronts exactly one identity; _source_platform
uses it (egress routing keeps _chat_platform unchanged).
Why: the same turn was `other/none` in task/session while delivery/latency said `discord`; an
unknown chat on a multi-platform connector was misattributed to the primary platform.
Tests: the existing unresolvable-chat stub now implements _metrics_platform (renamed hook). New:
task_start_fields({"platform": "relay"}) -> gateway_message/gateway/relay; unstamped chat on a
two-identity connector -> delivery platform `relay`. Both RED on the previous commit
(other/other/none; discord).
Probe (review/engagement/relay/probe_d_coverage.py), before: source.platform='relay' ->
{'entrypoint': 'other', 'execution_surface': 'other', 'platform': 'none'}, engagement surface None.
After: the contract test above.
Not done: resolving the turn-start task platform to the connector's platform (needs the adapter at
agent construction) and M6 (delivery row profile under multiplex); see NOT_COVERED.md.
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