[tmashara]

First real-world run of renderdiff: the Tailwind v3 → v4 migration of timlrx/tailwind-nextjs-starter-blog (PRs #1111 and #1122), last v3 commit vs post-migration commit, both production-built locally. 11 routes × 5 widths.

• docs/case-study.md: commands, trimmed real output from the first and final runs, the agent loop (fix → --rerun-failed → --accept), what it caught, what it got wrong, runtime.
• docs/img/case-study/: three real crops from run 1.

Headline: it caught two layout regressions the migration shipped (a 4px /about shift from v4's space-x selector change, still on upstream main, and a 4px header-nav shift at sm+), and it missed a visible brand-pink change because the pixel threshold treats it as the same colour. The "What it got wrong" section lists that miss and four rough edges (phantom bands, noisy dom properties, a flaky CSP error, unacceptable h-1 unaligned shots).

README is untouched.

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