[Task] apache-rat-plugin configured but never executed in CI + excldue typo

## Description
rat:check is bound to `verify` but CI stops at `test`, so it's dead. Separately `pom.xml:828` misspells `exclude` as `excldue` — Maven silently ignores it, so uppercase `.MD` files would be flagged as unlicensed if rat ever ran.

## Location
```
pom.xml:791-855 (rat bound to verify), pom.xml:828 (**/*.MD)
.github/workflows/ci.yml:125-137 (clean test — never reaches verify)
```

## Impact
License-file policy unenforced in PR CI; latent typo.

## Suggested fix
Run `mvn verify` (or `rat:check` explicitly) in `ci.yml`, or remove the dead rat config; fix `excldue`->`exclude`.

## Related existing issue(s)
#6083 is checkstyle-pom-license — this is a different mechanism (apache-rat-plugin) and the typo is unique.

_Identified during the 2026-08-02 audit; full list in [`docs/issue-candidates-2026-08-02.md`](docs/issue-candidates-2026-08-02.md)._

Read pom.xml:791-855 and .github/workflows/ci.yml:125-137 to confirm the RAT execution phase and the misspelled exclusion element. Run the CI Maven command locally or inspect its workflow behavior, then verify that RAT runs in PR CI, the exclusion is recognized, and the relevant Maven check passes.

Written by the indexing model from the issue text.