Bus input: serve envelopes from yukart/vehicle/+ and yukart/host/+

Newest per signal, stale after 5 s or when only the retained copy is known.
Bus or mock, never both; hello says which (source bus|mock|none).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

Cargo.toml

@@ -7,6 +7,7 @@ publish = false
 
 [dependencies]
 libc = "0.2"
+rumqttc = { version = "0.25", default-features = false }
 serde_json = "1"
 tungstenite = { version = "0.30", default-features = false, features = ["handshake"] }