[client] audio: recover physical underrun debt

A stable clock model can request a negative recovery slew while the
unbounded ring is physically underrun. This leaves the reader ahead of
the writer indefinitely and outputs silence until playback is restarted.

Floor active recovery at the physical low-water requirement. Rebase the
device-clock phase by any extra refill so modeled latency remains at the
target without restarting the underrun correction cycle.

Add a deterministic regression for negative physical debt opposing the
stable clock model.

client/tests/CMakeLists.txt

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