Next: per-route critical CSS via manifest v2 (P2.15)

[dbanksdesign]

Summary

• buildTypestylesForNext now traces each App Router page + layout chain, emits self-contained per-route CSS under app/_typestyles/routes/, and upgrades the extraction manifest to v2 with a routes map (enabled by default when app/ exists; opt out with routeCss: false).
• Adds getRouteCss(routePath, { root }) on @typestyles/next/server and @typestyles/build-runner to read pre-extracted route CSS at request time instead of the monolithic getRegisteredCss() buffer.
• Extends verifyTypestylesBuild with v2 manifest + per-route file checks; updates docs, next-app example verification, and changeset.

Test plan

• pnpm --filter @typestyles/build-runner test
• pnpm --filter @typestyles/next test
• Run pnpm next-app build and confirm app/typestyles.manifest.json is v2 with routes entries
• Confirm route CSS files exist under app/_typestyles/routes/

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