[common] audio: define the mediated LGMP stream protocol

Advertise optional audio mediation separately from runtime driver
availability, then negotiate ownership through the existing LGMP
client-control path.

Define fixed playback and capture stream records for packed PCM,
endpoint state, clock feedback, and targeted descriptor discovery.
Keep the LGMP queue table, recovery sidecar, and submodule unchanged.

client/include/interface/transport.h

@@ -68,6 +68,8 @@ enum
   LG_TRANSPORT_FEATURE_FRAME_SCHEDULE = 0x4,
   LG_TRANSPORT_FEATURE_INPUT          = 0x8,
   LG_TRANSPORT_FEATURE_CLIPBOARD      = 0x10,
+  /* The remote implements audio mediation; its driver may still be absent. */
+  LG_TRANSPORT_FEATURE_AUDIO          = 0x20,
 };
 
 typedef uint32_t LG_TransportFeatureFlags;