[client] audio: reset playback cleanly on resume

Count one underrun per starvation episode and end the sync diagnostics
epoch when a logical source stops.

Resume a compatible backend synchronously so data after a same-wire
restart is accepted immediately. Rebase the first post-idle device tick
without passing a long gap through the short-step clock filter.

Cancel in-flight backlog trims before fresh audio can be queued.

client/tests/CMakeLists.txt

@@ -821,6 +821,7 @@ if(ENABLE_AUDIO)
     playback-retry
     jitter
     recovery
+    resume
     consent
     quiesce
   )