Fix KaTeX parse errors in notebook math blocks

[KaynXu]

Summary

• Split single-line display math blocks into multiline $$ blocks in LA_01_01_01.ipynb and LA_01_07_03.ipynb.
• Keeps the original formulas unchanged while avoiding KaTeX parsing later inline $...$ expressions as part of the same math block.

Verification

Temporary proof script: /tmp/verify-katex-XXXXXX.js
Notebook files parsed: 88
Risky single-line $$...$$ lines remaining: 0

Fixes #78