Label Matins nocturns and the Invitatory in the rendered ordo

Adds a section-heading ResolvedPart kind and emits "Invitatory" and
"Nocturn I/II/III" headings in matins.ts, so the hour's structural
boundaries are visible instead of rendering as one undifferentiated
stream of parts. Ferial (1-nocturn) days get no Nocturn heading, matching
their existing single-unnumbered-nocturn design.

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

src/ui/hour-view.ts

@@ -122,6 +122,8 @@ function renderPart(part: ResolvedPart, languages: readonly string[]): string {
           ${part.psalms.map((p) => renderPart(p, languages)).join('')}
         </section>
       `;
+    case 'section-heading':
+      return `<h2 class="section-heading">${escapeHtml(part.label)}</h2>`;
     case 'te-deum':
       return `
         <section class="ordo-part ordo-part-te-deum">