perf(flex): optimize flex measurement and relayout boundaries

[andycall]

Summary

• skip speculative flex run-metric work for simple rows and reuse safe flow/text-heavy measurements during _computeRunMetrics()
• establish tighter relayout boundaries through flex/widget wrapper layout paths and move intrinsic-measurement invalidation onto targeted style/content changes
• fix centered column flex scroll overflow to use aligned child offsets and add focused regression coverage
• add opt-in flex profiling flags/helpers used to diagnose fast-path and anonymous/text-heavy measurement reuse

Testing

• cd integration_tests && flutter build macos --debug
• cd integration_tests && npm run integration -- --skip-build specs/css/css-flexbox/flex-grow-chat-layout-issue-520.ts specs/css/css-flexbox/flex-shrink-001.ts
• Focused rebuilt integration slices were run during development for popup/widget flex, anonymous/text-wrapper flex, pseudo-hover-active, translate, and dynamic-bsize regressions

Notes

• This PR includes only committed changes on feat/layout_optimize through c6c07bd2b
• Additional local worktree changes in this checkout are not part of the PR

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Project	Deployment	Actions	Updated (UTC)
use-case	Ready	Preview, Comment	Mar 22, 2026 7:27pm
vue_usecase	Ready	Preview, Comment	Mar 22, 2026 7:27pm