Add budget tracker example

[beta-devin-ai-integration]

Summary

Adds a new examples/budget/ example that builds a monthly budget tracker comparing budgeted vs. actual spending across 8 categories, with a summary table. This fills coverage gaps for library features not demonstrated in existing examples:

• el.sum — used to total budgeted/actual amounts and count over-budget categories
• el.if_ — flags whether each category is at or over budget
• >= comparison operator — previously only > was shown (in the options example)

Also demonstrates cross-frame references (from_=budget_df), display_horizontally() on the summary, editable cells, and accounting-style currency formatting.

Updates examples/README.md with a description of the new example.

Review & Testing Checklist for Human

• The over_budget column uses >= (not >), so categories where actual exactly equals budgeted are flagged as "over." Verify this is the desired semantics — e.g., Rent ($1500/$1500) and Savings ($500/$500) will show over_budget = 1.
• Run excelify-viewer --file-path examples/budget/main.py to visually verify the layout, formatting, and that editable cells work as expected when values are changed.

Notes

• Pre-commit hooks (including pyright) pass. The script runs without errors.
• No tests added — consistent with existing examples which also have no tests.
• Link to Devin run
• Requested by: @alberthan-windsurf

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[beta-devin-ai-integration]

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