test: assert the lean engine module stays free of Apache POI

[DemchaAV]

Why

#298 extracted the semantic DOCX/PPTX backend and Apache POI into graph-compose-render-docx, leaving the engine (root module) POI-free. Nothing machine-guards that invariant now: the old no-poi CI job was removed, and even when it ran it only executed the suite without POI on the classpath — it never asserted POI was absent from the sources. A future change that re-adds poi-ooxml to the engine pom (directly or transitively) and imports it would compile clean, silently pulling POI back into the lean core.

What changed

Adds EnginePoiIsolationGuardTest (test scope, com.demcha.documentation, next to PdfBackendIsolationGuardTest). It walks src/main/java and src/test/java under com/demcha/compose and fails on any import org.apache.poi. or fully-qualified org.apache.poi. reference, mirroring PdfBackendIsolationGuardTest's grep-style walk/assert. Unlike the PDFBox sibling — which permits PDFBox under backend.fixed.pdf — this guard allows POI nowhere in the engine. Test-only; no production code changes.

Verification

• ./mvnw test -Dtest=EnginePoiIsolationGuardTest -pl . → green (1 test).
• ./mvnw verify -pl . → BUILD SUCCESS, 1368 tests, 0 failures (javadoc gate included).
• Proven the guard bites, not just the compiler: temporarily put poi-ooxml:5.5.1 back on the engine classpath and added import org.apache.poi.xwpf.usermodel.XWPFDocument; to GraphCompose.java so it compiled; the guard then failed at its assertion — Expecting empty but was: {GraphCompose.java=[org.apache.poi.xwpf.usermodel.XWPFDocument]} — and went green again after reverting both edits.