Fix malgo concurrent device initialization crash

[akj]

Summary

• Serializes malgo.InitDevice calls against the shared malgo context.
• Prevents host backends such as PulseAudio from aborting inside ma_device_init when concurrent playback tests initialize devices at the same time.
• Keeps playback callbacks concurrent after device initialization; only the narrow C-side init section is locked.

Test plan

• go test ./internal/audio/malgo -count=3
• go test ./...
• CGO_ENABLED=0 go test ./...