[MISSING_README] False positive when migrate spec to FSv2

[mikeharder]

Fails with MISSING_README when migrating a spec from FSv1 to FSv2:

https://github.com/Azure/azure-rest-api-specs/actions/runs/17168210019/job/48712929271#step:4:38

I suspect the root cause is a bug / invalid assumption in our before/after diff logic. If a PR moves or deletes a readme.md that existed in "before", it can cause MISSING_README, even if the "after" state is completely valid.

Possibly related: #151

Repro

Azure/azure-rest-api-specs#37991
https://github.com/Azure/azure-rest-api-specs/actions/runs/18359705958

// setup
gh repo clone Azure/azure-rest-api-specs
git fetch upstream refs/pull/37991/merge
git checkout -b pr-37791 FETCH_HEAD
export SYSTEM_PULLREQUEST_TARGETBRANCH=main
export TRAVIS_PULL_REQUEST_SHA=2efb7e1ea80da4021d63f36409fdbb9a068b5cfc
export TRAVIS_REPO_SLUG=Azure/azure-rest-api-specs

// reset
gb -d source-b6791c5f-e0a5-49b1-9175-d7fd3e341cb8 main
rm -rf /home/mharder/c93b354fd9c14905bb574a8834c4d69b

// run
  npm exec --no -- avocado \
    --excludePaths \
      "/common-types/" \
      "/scenarios/" \
      "/package.json" \
      "/package-lock.json" \
      "/cadl/examples/" \
      '(?=/examples/)(?!(?:/stable/|/preview/))' \
      "/\\.github/" \
      "/eng/" \
    --includePaths \
      "data-plane" \
      "resource-manager"