feat: modernize React Memoization chapter

[davidbarna]

Summary

• Update React.memo → memo() named import style (React 18+)
• Convert all code snippets from JS/JSX to TypeScript/TSX with type annotations
• Replace curried event handler example with idiomatic name-attribute pattern
• Add fiber node caveats to simplified useCallback/useMemo implementations
• Expand "when to use / when not to" section with individual slides and code examples for each bullet point
• Add React Compiler section covering auto-memoization in React 19
• Restructure intro into "Equality in JavaScript" and "Functions are objects" sections

Test plan

• Run npm run build and verify Gatsby renders the chapter without errors
• Open the chapter in the browser and verify all slides render correctly
• Check that code syntax highlighting works for ts and tsx fences

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