Add a PipeWire audio backend and stereo capture/playback

Capture and playback now go through AudioCapture/AudioPlayback interfaces
with two implementations: PipeWire, used for the system default and for
every pw: device when a session is reachable, and Java Sound for the raw
ALSA devices and other platforms. The PipeWire binding is FFM-based
(PipeWireLibrary, PipeWireSession, PipeWireStream, SpaPod) and enumerates
the graph's real devices, so each stream shows up separately in volume
mixers.

Lines are opened with a negotiated channel count instead of a fixed mono
format: OPUS_MUSIC transmits the stereo capture as-is, OPUS_VOICE keeps
transmitting the mono downmix so the pre-processing chain and VAD still
see a single channel. Java Sound enumeration asks only for "a mixer that
can capture/play" so the PipeWire and PulseAudio ALSA plugins are no
longer hidden by their narrow format lists.

The Java Sound classes are renamed to Desktop* to match what they now
are, and JUnit plus a SpaPod round-trip test are wired into the build.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

ts3-client/desktop/pom.xml

@@ -12,7 +12,7 @@
 
     <artifactId>ts3-client-desktop</artifactId>
     <name>TS3J Client Desktop Audio</name>
-    <description>Desktop audio backend: Java Sound capture/playback and native Opus via the FFM API</description>
+    <description>Desktop audio backend: PipeWire/Java Sound capture/playback and native Opus via the FFM API</description>
 
     <dependencies>
         <dependency>
@@ -23,6 +23,12 @@
             <groupId>com.github.manevolent</groupId>
             <artifactId>ts3j</artifactId>
         </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter</artifactId>
+            <version>5.13.4</version>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
     <build>