refactor(app-router): build discoverSiblingInterceptingRoutes paths with path.posix

[shulaoda]

What

In discoverSiblingInterceptingRoutes, derive the owner and child directories with path.posix.*, and document that appDir and each route's pagePath / routePath must be forward-slash:

• routeDir: normalizePathSeparators(path.dirname(filePath)) → path.posix.dirname(filePath).
• childDir: path.join(dir, entry.name) → path.posix.join(dir, entry.name).

Why

A route's pagePath / routePath is now a forward-slash canonical id, and appDir is forward-slash by the route-graph contract. So path.posix.dirname(filePath) yields the forward-slash owner directory directly — the old normalizePathSeparators(path.dirname(...)) only existed because filePath used to be native (backslash) on Windows. findOwnerRouteForDir compares in forward-slash space, so routeDir must be forward-slash, which it now is without the normalization pass. The marker-scan childDir switches to path.posix.join for the same reason.

How

• routeDir uses path.posix.dirname(filePath) (drops normalizePathSeparators and the now-stale comment).
• childDir uses path.posix.join(dir, entry.name).
• Doc note: appDir / pagePath / routePath must be forward-slash; owner dirs come from pagePath / routePath matched against appDir-relative path.posix.* paths.

Testing

• vp check packages/vinext/src/routing/app-route-graph.ts — format, lint, and typecheck clean.
• No-op on POSIX (path.posix === path). On Windows, given the forward-slash pagePath / routePath / appDir, routeDir and childDir are identical to the previous normalized results. Behavior-preserving, hence refactor.

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commit: ba41bdb

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Performance benchmarks

Compared ba41bdb against base c8aef02 using alternating same-runner rounds. Next.js was unchanged and skipped.

0 improved · 1 regressed · 5 within ±1.5%

Scenario	Framework	Baseline	Current	Change
Client bundle size (gzip)	vinext	124.0 KB	124.0 KB	⚫ +0.0%
Client entry size (gzip)	vinext	118.8 KB	118.8 KB	⚫ +0.0%
Dev server cold start	vinext	1.94 s	1.96 s	⚫ +1.0%
Production build time	vinext	2.28 s	2.32 s	🔴 +1.6%
RSC entry closure size (gzip)	vinext	95.2 KB	95.2 KB	⚫ +0.0%
Server bundle size (gzip)	vinext	161.5 KB	161.5 KB	⚫ +0.0%

View detailed results and traces

_{🟢 improvement · 🔴 regression · ⚫ change below 1.5% · paired base/head}