Add benchmarks and optimize hot paths

[nstepien]

Summary

• Add vitest bench infrastructure with browser-based render benchmarks, hook-level benchmarks (useViewportRows, useCalculatedColumns), and utility micro-benchmarks
• Memoize HeaderCell with React.memo, pre-compute sort info via useMemo Map in HeaderRow, and stabilize callback identity with useLatestFunc — eliminating per-cell O(n) findIndex scans and unnecessary re-renders
• Lazy-copy templateColumns array in useColumnWidths (only spread when modifications are needed)
• Replace columnMetrics Map with plain array in useCalculatedColumns for faster index-based access in viewport calculation

Test plan

• All 317 functional tests pass
• TypeScript compiles cleanly
• ESLint passes
• Benchmarks run successfully via npm run bench

🤖 Generated with Claude Code

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.52%. Comparing base (e533510) to head (7e0f9b5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3995      +/-   ##
==========================================
+ Coverage   97.51%   97.52%   +0.01%     
==========================================
  Files          36       36              
  Lines        1449     1456       +7     
  Branches      463      468       +5     
==========================================
+ Hits         1413     1420       +7     
  Misses         36       36              

Files with missing lines	Coverage Δ
src/HeaderCell.tsx	94.44% <100.00%> (-1.34%)	⬇️
src/HeaderRow.tsx	100.00% <100.00%> (ø)
src/hooks/useCalculatedColumns.ts	100.00% <100.00%> (ø)
src/hooks/useColumnWidths.ts	98.63% <100.00%> (+0.01%)	⬆️

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