The docx package always emits its own Title and Heading1-6 styles; our
same-id copies in paragraphStyles made styles.xml define them twice, and
both Word and LibreOffice then dropped the basedOn chain — headings
rendered serif, regular, at the raw size. buildStyles now feeds those
seven through styles.default.title/headingN, leaving a single
definition with the chain intact. Regression test asserts unique
styleIds and Heading1's basedOn.