[codex] Normalize routing paths for Windows

[WilliamK112]

Summary

Part of #1605.

• normalize file scanner and convention-file probe paths to forward slashes for internal routing use
• keep Pages Router and App Router route construction in forward-slash path space on Windows
• update route graph helpers/tests so pagePath, layoutPath, errorPath, slot paths, and scanner output are validated with normalized separators

Validation

• corepack pnpm test tests/windows-path-normalization.test.ts tests/app-route-graph.test.ts tests/routing.test.ts tests/route-sorting.test.ts
• corepack pnpm run fmt:check
• corepack pnpm run check

[WilliamK112]

Closing this stale draft as superseded by #2502, which covers the broader Windows path normalization/pathslash migration and is from the contributor who took #1605 in the issue thread. This branch is also now conflicting with main, so keeping it open would add duplicate review noise.