fix: include <sstream> in audio cell of demo notebook

[mvanhorn]

Closes #480.

Adds #include <sstream> to the audio cell of test/Notebooks/xeus-cpp.ipynb. The cell uses std::stringstream (au::audio::m_buffer) without the corresponding header, so toolchains that don't transitively include <sstream> via <string>/<fstream> fail to compile the cell, which is what produced the failure in #480.

The companion wasm demo notebook (referenced by @anutosh491 in the issue) already has the include — this brings the desktop demo in line.

Single-line, JSON-valid edit (the notebook still parses cleanly with json.load).

[JohanMabille]

Thanks for the fix!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.51%. Comparing base (88a82c5) to head (0d90f4d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #481      +/-   ##
==========================================
- Coverage   74.25%   73.51%   -0.74%     
==========================================
  Files          23       23              
  Lines        1301     1193     -108     
  Branches      111      111              
==========================================
- Hits          966      877      -89     
+ Misses        335      316      -19     

see 16 files with indirect coverage changes
see 16 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.