test: add end-to-end epub test for structured subject metadata

Verifies that the consolidated subject schema in document-metadata.yml
correctly renders structured subject values (text/authority/term) into
EPUB3 OPF metadata.

Author: gordonwoodhull

tests/docs/smoke-all/2026/03/01/subject-structured-epub.qmd

@@ -0,0 +1,18 @@
+---
+title: Test
+subject:
+  text: "Science"
+  authority: "BISAC"
+  term: "SCI000000"
+format: epub
+_quarto:
+  tests:
+    epub:
+      ensureEpubFileRegexMatches:
+        - path: EPUB/content.opf
+          regexes:
+            - ['dc:subject id="subject-1">Science</dc:subject>', 'property="authority">BISAC', 'property="term">SCI000000']
+            - []
+---
+
+Test structured subject metadata in epub output.