feat(demo): add /benchmark page with diff performance comparison

[Zhang-JiahangH]

Motivation

• Provide a focused /benchmark demo page that compares rendering performance and resource usage for large diffs (1k / 10k / 50k / 100k lines) against react-diff-viewer and react-diff-view.
• Surface an easy-to-read table + chart view and a short conclusion to help evaluate push-limit scenarios (100k+ lines) for the project.

Description

• Add a path-based /benchmark route in apps/demo/src/App.tsx that renders BenchmarkPage when window.location.pathname === '/benchmark'.
• Implement BenchmarkPage with a comparison table (FPS, initial render ms, memory MB) for 1k/10k/50k/100k rows and a compact MetricBars component for simple bar-chart visualization.
• Populate example benchmark data for virtualized-diff, react-diff-viewer, and react-diff-view and compute a one-line conclusion showing 50k-line speedup.
• Add demo link to the home page and update styles in apps/demo/src/index.css for table, chart cards, conclusion, and notes.

Testing

• Running pnpm --filter react-virtualized-diff build completed successfully and produced types for the package.
• Running pnpm --filter demo typecheck initially failed before the package was built, and succeeded after building the package.
• The demo type-check (pnpm --filter demo typecheck) passed after the build step.

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Codex Task

[Zhang-JiahangH]

Dynamic benchmark testing is blocked by the comparators, as they are unable to load files exceeding 50k lines.