Phase 3 of relay-channel-context (gateway/agent side, single PR). The
connector (gateway-gateway #122/#123/#124) now attaches read-only
surrounding channel/group context to an addressed relay turn; this wires
the gateway to consume it.
- descriptor.py: additive optional supports_context (default False) on
CapabilityDescriptor. from_json already filters unknown keys, so this is
back-compat both directions within contract_version 1.
- ws_transport.py: _event_from_wire maps the connector's read-only
context[] array into the EXISTING MessageEvent.channel_context field via
a new _render_relay_context() helper — reusing the same read-only
injection path history-backfill uses (run.py prepends channel_context
ahead of the trigger message). Never raises; absent/empty/malformed ->
channel_context unset (byte-identical to today).
- docs/relay-connector-contract.md: document supports_context in the §2
descriptor table (fixes the contract-doc conformance test) + the
context/context_error inbound fields in §3.
- tests: descriptor default/round-trip/forward-compat; _render_relay_context
rendering + malformed-safe; _event_from_wire context->channel_context
mapping + the read-only invariant (trigger text untouched).
RELAY-ONLY: only gateway/relay/* + the shared MessageEvent consumption via
its existing channel_context field. No native adapter touched.
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