Pending DM key exchange: QueueStatus.state = KEY_EXCHANGE (protobufs#1020)

[jamesarich]

Tracks the Android side of meshtastic/protobufs#1020 (open, no reviews yet). Additive QueueStatus.state (STATE_UNSPECIFIED, KEY_EXCHANGE), reported against the original mesh_packet_id while firmware fetches a missing recipient key.

Android already routes QueueStatus through FromRadioPacketHandlerImpl.handleQueueStatus (core/data/.../FromRadioPacketHandlerImpl.kt:116).

• blocked on protobufs#1020 merge + protobufs bump
• map state == KEY_EXCHANGE onto the existing pending message row by mesh_packet_id (new message status, e.g. "Exchanging keys…") — no retry, no duplicate ID
• wording aligned with the cross-client message-status table in [ALIGNMENT]: Message Status design#43