Add visualization and convergence demonstration to fitted VFI lecture (#711)

- Add vertical black line to plot showing reservation wage w_bar
- Fix markdown formatting issue with empty code cell
- Add demonstration showing how increasing simulation length (T=10,000) reduces difference between time-average and cross-sectional unemployment rates from 0.0594 to 0.0016
- Reorganize sections for better flow and clarity
- Improve code structure and documentation

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Co-authored-by: Claude <noreply@anthropic.com>

Author: jstac