fix(test): M83 walkFiles path assertions on Windows

[Roarpeng]

Why

main @ c46a9d7 (v1.18.3) failed validate-platforms (windows-latest):

tests/m83-index-scan-budget.test.ts
  expected [ Array(1) ] to deeply equal [ 'src/app.ts' ]
  + "C:\\Users\\RUNNER~1\\AppData\\Local\\Temp\\gf-m83-nogit-...\\src\\app.ts"

walkFiles returns absolute paths. The tests stripped root + "/", which never matches root + "\\" on Windows, so the assertion saw the full path.

Changes

Use the same conversion as walkScannableFiles: normalizePath(relative(root, absPath)). No walker API change.

Note

One Build run also failed validate (ubuntu-latest, 20) with ERR_IPC_CHANNEL_CLOSED. That looks like the new index worker-pool flake, not this path bug; other CI runs on the same commit had ubuntu-20 green.

Validation

npx vitest run tests/m83-index-scan-budget.test.ts — 6 passed (Linux). Windows coverage is the CI job this PR is for.