CapabilityDescriptor.from_platform_entry() projects an existing PlatformEntry
(label, max_message_length, emoji, platform_hint, pii_safe, name) into a
descriptor, proving the descriptor is a projection of existing config rather
than a parallel concept. Runtime-only capabilities (len_unit, draft/edit/
thread/markdown) are caller-supplied. max_message_length==0 ('no limit') maps
to the stream_consumer 4096 default.
Phase 0 complete. Task 0.3 of the gateway-relay plan.