fix: route Title/Heading1-6 through the docx factory's override slots

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.

src/lib/export/CLAUDE.md

@@ -54,6 +54,8 @@ fallback download shows a one-time hint (`edentext-download-hint`) that the brow
 - **`normalizeColor`** — coerces colors to `#RRGGBB` (ODF requirement; rejects/normalizes `rgb()` and short hex).
 - **Schema conformance** (guarded by `tests/schema-validation.test.ts`; LibreOffice forgives all of this, Word's strict reader does not): `applyOdfVersion` stamps every ODF part **1.3** — the version LibreOffice writes, and the first with `style:header-first` — over odf-kit's 1.2. `draw:image` carries the `xlink:type/show/actuate` trio (`xlink:type` is mandatory beside `xlink:href`); `text:time-value` is an xsd dateTime, never a `PT…S` duration (the ODT importer still reads the legacy duration); `index-entry-link-start/-end` only in TOC entry templates; `text:dont-balance-text-columns` on `style:section-properties`. DOCX: `orderDocxSettings` (last pass) re-sorts `w:settings` children into the fixed CT_Settings sequence the prepend-passes scramble; `w14:paraId` in comments.xml requires `mc:Ignorable="w14"` on the root; `styleXmlComponent` unwraps `ImportedXmlComponent.fromXmlString`'s nameless wrapper, which would serialize as a literal `<undefined>` element.
 
+- **Factory style slots** (`FACTORY_SLOTS`, `buildStyles`): the `docx` package always writes its own Title/Heading1–6 into styles.xml, so the registry's versions ride `styles.default.title/headingN` instead of `paragraphStyles` — a second definition under the same `w:styleId` makes Word **and** LibreOffice drop the `basedOn` chain (headings lose their sans/bold).
+
 The filename is derived from the first non-empty heading (max 50 chars, sanitized), falling back to `document.odt`.
 
 ## Header/footer export

src/lib/export/docx.ts

@@ -2412,15 +2412,33 @@ function numberingStyleXml(name: string): ImportedXmlComponent {
   );
 }
 
+// Styles the library always writes itself; ours must ride its override slots — a second
+// definition under the same styleId makes Word and LibreOffice drop the basedOn chain.
+const FACTORY_SLOTS: Record<string, string> = {
+  Title: 'title', Heading1: 'heading1', Heading2: 'heading2', Heading3: 'heading3',
+  Heading4: 'heading4', Heading5: 'heading5', Heading6: 'heading6',
+};
+
 function buildStyles(sheet: StyleSheet, used: Set<string>, language?: { language: string; country: string } | null, usedTables: string[] = [], usedLists: string[] = []) {
   const run: Writable<IRunStylePropertiesOptions> = { font: DOC_FONT, size: 24 };
   if (language) run.language = { value: `${language.language}-${language.country}` };
+  const slotted: Record<string, Omit<IParagraphStyleOptions, 'id' | 'name'>> = {};
+  const paragraphStyles = Object.values(sheet.paragraph)
+    .filter(st => used.has(st.name)).map(paragraphStyleOf)
+    .filter((st) => {
+      const slot = FACTORY_SLOTS[st.id];
+      if (!slot) return true;
+      const { id: _id, name: _name, ...rest } = st;
+      slotted[slot] = rest;
+      return false;
+    });
   return {
     default: {
       document: { run, paragraph: { spacing: { after: 0, line: 240, lineRule: LineRuleType.AUTO } } },
+      ...slotted,
     },
     // The document's named styles, chain intact — Word shows them in its style list.
-    paragraphStyles: Object.values(sheet.paragraph).filter(st => used.has(st.name)).map(paragraphStyleOf),
+    paragraphStyles,
     characterStyles: Object.values(sheet.character ?? {}).map(characterStyleOf),
     ...(usedTables.length || usedLists.length
       ? { importedStyles: [...usedTables.map(tableStyleXml), ...usedLists.map(numberingStyleXml)] }

tests/docx-roundtrip.test.ts

@@ -160,6 +160,16 @@ describe('DOCX export → import round trip', () => {
     expect(table.content![0].content![0].attrs.backgroundColor).toBe('#4A7EBB');
   });
 
+  it('defines Title and Heading1-6 exactly once, chain intact', async () => {
+    const doc = { type: 'doc', content: [heading(1, 'Kapitel'), para('Text')] } as any;
+    const files = unzipSync(await buildDocx(doc));
+    const stylesXml = strFromU8(files['word/styles.xml']);
+    const ids = [...stylesXml.matchAll(/w:styleId="([^"]+)"/g)].map((m) => m[1]);
+    // A second definition under the same id makes Word and LO drop the basedOn chain.
+    expect(ids.length).toBe(new Set(ids).size);
+    expect(stylesXml).toMatch(/w:styleId="Heading1"><w:name w:val="Heading 1"\/><w:basedOn w:val="Heading"\/>/);
+  });
+
   it('round-trips the table style options as w:tblLook', async () => {
     const cell = (t: string): N => ({
       type: 'tableCell', attrs: { colspan: 1, rowspan: 1, colwidth: null }, content: [para(t)],