Implement GameCube/Wii audio playback via libansnd

Allocates an ansnd voice per stream and configures/starts it in
single-buffer mode (no continuous re-feed needed yet, matching the
PSP/Linux single-shot approach). Real linear panning from
directionality via left/right voice volume. ansnd has no polling API
for voice state, so finish detection uses a voice_callback (fired
from ansnd's own audio DMA interrupt, not the main thread) that sets
a flag audioStreamDolphinIsFinished() reads.

Links libansnd (new for this project) and wires duskdolphin/audio
into the CMake build for the first time.

Verified via the dusk-dolphin toolchain: both GameCube and Wii
targets compile and link cleanly. Runtime verification in headless
Dolphin was inconclusive - confirmed via an A/B test against a
pre-audio baseline build that a pre-existing MMIO flood (unrelated
to this change, reproduces identically with zero audio code present)
keeps the harness from reaching the game's own steady state before
timing out.

cmake/targets/dolphin.cmake

@@ -61,6 +61,7 @@ target_link_libraries(${DUSK_LIBRARY_TARGET_NAME} PRIVATE
   zstd
   z
   lzma
+  ansnd
 )
 
 if(DUSK_DOLPHIN_BUILD_TYPE STREQUAL "ISO")