Wire up the duplicate-classes-integration-test

The standalone Maven project was never invoked by CI or pytest.
Move into tests/fixtures alongside the other BOM fixtures, and add
a real integration test that runs it through mega-melt validation,
confirming pom-scijava's banDuplicateClasses enforcer rule
actually fires when log4j and reload4j collide on the classpath.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: ctrueden

…plicate-classes-integration-test/pom.xml tests/fixtures/duplicate-classes/pom.xmlduplicate-classes-integration-test/pom.xml renamed to tests/fixtures/duplicate-classes/pom.xml duplicate-classes-integration-test/pom.xml renamed to tests/fixtures/duplicate-classes/pom.xml

@@ -0,0 +1,46 @@
+"""Integration test for mega-melt's duplicate-class detection.
+
+Uses the duplicate-classes fixture (a POM that depends directly on both
+log4j and reload4j, which both provide org.apache.log4j.* classes) to
+confirm that pom-scijava's banDuplicateClasses enforcer rule — the rule
+mega-melt validation relies on to catch classpath conflicts across a BOM
+(see README's mega-melt section) — actually fires.
+
+This runs a real Maven build against Maven Central and is intentionally
+slow.
+"""
+
+from __future__ import annotations
+
+import shutil
+from pathlib import Path
+
+from pombast.maven._builder import locate_java
+from pombast.maven._mega_melt import run_mega_melt_validation
+
+FIXTURE = Path(__file__).parent.parent / "fixtures" / "duplicate-classes"
+
+# pom-scijava's parent chain requires Java 11+; pin it explicitly so this
+# test doesn't spuriously fail on an older ambient JDK before ever reaching
+# the duplicate-classes rule.
+_JAVA_VERSION = 11
+
+
+def test_duplicate_classes_fail_enforcer(tmp_path):
+    """log4j + reload4j together should trip banDuplicateClasses."""
+    mega_melt_dir = tmp_path / "duplicate-classes"
+    shutil.copytree(FIXTURE, mega_melt_dir)
+
+    java_home = locate_java(_JAVA_VERSION)
+    success, _tree_log, build_log = run_mega_melt_validation(
+        mega_melt_dir, java_home=java_home
+    )
+
+    assert not success, "expected enforcer to fail on duplicate classes"
+
+    log_content = build_log.read_text(encoding="utf-8")
+    assert "BanDuplicateClasses" in log_content, (
+        f"expected a BanDuplicateClasses enforcer failure, but got:\n{log_content}"
+    )
+    assert "ch.qos.reload4j:reload4j" in log_content
+    assert "log4j:log4j" in log_content