fix: add missing sample.wav test file for decoder-audio integration tests (#131)

- Update .gitignore to allow sample.wav (previously all .wav files were ignored)
- Add sample.wav test file required by integration tests

The integration tests expect sample.wav to test WAV format decoding, but it was
excluded by gitignore rules during monorepo migration.

Author: sharmaraju352

packages/qvac-lib-decoder-audio/.gitignore

@@ -9,3 +9,4 @@ coverage
 # Generated example outputs
 example/*.raw
 example/*.wav
+!example/sample.wav