Add benchmark analysis with cost analysis

[jathavaan]

Summary

• Refactored benchmark analysis into separate RQ1 and RQ2 notebooks with full statistical tests, visualisations, and LaTeX table exports
• Added infrastructure cost analysis (cost bars, cost breakdown by category, cost-vs-performance scatter) to both notebooks using the already-loaded but previously unused costs_df
• New functions: enrich_costs(), build_cost_summary(), format_cost_table(), plot_cost_bars(), plot_cost_breakdown(), plot_cost_performance()

Test plan

• Run rq1-analysis.ipynb end-to-end and verify cost figures generate
• Run rq2-analysis.ipynb end-to-end and verify cost figures generate
• Confirm rq1_cost_summary.tex and rq2_cost_summary.tex render correctly in LaTeX

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