fix(audio): add missing using-declarations for the mixer bindings

NodeAddon.cpp imports each addon-namespace binding explicitly; the mixer
exports compiled in DeviceBindings but were undeclared at the export
table. Full MSVC build:audio now green; addon-exports snapshot
regenerated (7 new exports).

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

src/audio/NodeAddon.cpp

@@ -118,6 +118,13 @@ using slopsmith::addon::SetPan;
 using slopsmith::addon::SetParameter;
 using slopsmith::addon::SetPostGain;
 using slopsmith::addon::SetRendererBus;
+using slopsmith::addon::MixerCreateChannel;
+using slopsmith::addon::MixerReleaseChannel;
+using slopsmith::addon::MixerPushChannel;
+using slopsmith::addon::MixerSetChannelGain;
+using slopsmith::addon::MixerSetChannelMute;
+using slopsmith::addon::MixerSetChannelGroup;
+using slopsmith::addon::MixerListChannels;
 using slopsmith::addon::SetSlotState;
 using slopsmith::addon::SetSourceChart;
 using slopsmith::addon::SetSourceInputChannel;

tests/contracts/addon-exports.json

@@ -51,6 +51,13 @@
   "loadPluginList",
   "loadPreset",
   "loadVST",
+  "mixerCreateChannel",
+  "mixerListChannels",
+  "mixerPushChannel",
+  "mixerReleaseChannel",
+  "mixerSetChannelGain",
+  "mixerSetChannelGroup",
+  "mixerSetChannelMute",
   "moveProcessor",
   "openPluginEditor",
   "probeDeviceOptions",