docs: note honest date formatter, title formatter and drift fixture

Documents DocumentTitleFormatter in the document-management C4 diagram and adds
an "honest precision display" row to the CONTRIBUTING date-handling table,
pointing at formatDocumentDate / <DocumentDate>, the shared
docs/date-label-fixtures.json drift guard, and the {@html} escaping rule.

Closes #666

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CONTRIBUTING.md

@@ -272,6 +272,7 @@ For multipart/form-data (file uploads): bypass the typed client and use `event.f
 | Form display | German `dd.mm.yyyy` with auto-dot insertion via `handleDateInput()` |
 | Wire format | ISO 8601 via a hidden `<input type="hidden" name="documentDate" value={dateIso}>` |
 | Display | `new Intl.DateTimeFormat('de-DE', …).format(new Date(val + 'T12:00:00'))` |
+| Honest precision display | `formatDocumentDate(iso, precision, end?, raw?, locale?)` (`$lib/shared/utils/documentDate.ts`) or the `<DocumentDate>` component — renders a document date at exactly its `meta_date_precision` (MONTH → "Juni 1916", never a fabricated day). It mirrors the Java `DocumentTitleFormatter`; both are pinned to `docs/date-label-fixtures.json` so the title and UI labels can't drift. `meta_date_raw` is untrusted — render it via default escaping, never `{@html}` (a CI guard enforces this). |
 
 ### Security checklist (new endpoint)