RelayAdapter.supports_inchannel_continuable is a scalar adopted from the
PRIMARY identity's handshake descriptor, but one RelayAdapter fronts N
platforms and the connector advertises the bit per platform. Reading the
scalar for every logical platform both leaked a Slack-primary True onto
other fronted platforms (activating the flat surface their descriptor
never advertised) and suppressed a non-primary platform's advertised
True (forcing thread mode on capable Slack behind a Discord primary).
Add supports_inchannel_continuable_for_platform(platform): resolves the
platform's own negotiated descriptor via descriptor_for_platform (the
same Phase 1.5 seam max_message_length uses), scalar fallback only when
the per-platform descriptor is unavailable. The scheduler's D6 gate
prefers the query when the adapter provides it; native adapters keep
the class-attribute path byte-identically.
Tests: two-platform descriptor matrix (primary-True no-leak,
non-primary-True honored, unknown-platform scalar fallback).
Ben Barclay
162b23c3e2