One BasePlatformAdapter subclass that reads its capability profile from a
CapabilityDescriptor: MAX_MESSAGE_LENGTH attribute, message_len_fn (table-driven
by len_unit: chars=len, utf16=Telegram-style code units), supports_draft_streaming.
Implements the four abstract methods (connect/disconnect/send/get_chat_info) by
delegating to an injected RelayTransport (full protocol lands in Task 1.2). Adds
Platform.RELAY enum member. No per-platform gateway code.
Phase 1, Task 1.1 of the gateway-relay plan.
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