Fix excess whitespace in streaming output from code cells

[mmcky]

Summary

Fixes #325 - Removes excess whitespace between consecutive streaming output lines from Jupyter code cells.

Problem

When Jupyter cells produce streaming output (e.g., multiple print statements), MyST-NB renders each output as a separate .output.stream element with margin-top: 1em, creating excessive vertical spacing between lines.

Solution

Added CSS rule to remove top margin from consecutive streaming output elements:

// Fix excess spacing in streaming output (Issue #325)
.cell_output .output.stream + .output.stream {
  margin-top: 0;
}

This uses the adjacent sibling selector (+) to target only streaming outputs that immediately follow another streaming output, preserving normal spacing for other output types.

Changes

• src/quantecon_book_theme/assets/styles/index.scss: Added CSS fix for streaming output spacing
• tests/sites/base/section1/ntbk_streaming.ipynb: Created test notebook with streaming output examples
• tests/sites/base/section1/index.md: Added test notebook to toctree
• tests/test_build.py: Added test_streaming_output_spacing() function
• tests/test_build/test_build_book.html: Updated regression test baseline

Testing

• ✅ All 12 tests pass on Python 3.12 and 3.13
• ✅ New test verifies streaming output structure builds correctly
• ✅ Regression tests updated and passing

Build Commands Used

npm run build           # Compile SCSS assets
tox                     # Run full test suite

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@67f6658). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #330   +/-   ##
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  Coverage        ?   20.89%           
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  Files           ?        2           
  Lines           ?      292           
  Branches        ?        0           
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  Hits            ?       61           
  Misses          ?      231           
  Partials        ?        0           

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[mmcky]

[mmcky]

@jstac this is where we are up to. Still too much whitespace but at least it is now consistent (hitting a css barrier that I need to think through)

[mmcky]

This approach QuantEcon/lecture-python.myst#725 is better than fixing in the them.