PPTX: keep custom XML, the sensitivity label and custom properties

Decks from SharePoint and Microsoft 365 carry customXml items (document
library metadata and content types), docMetadata/LabelInfo.xml (the
Microsoft Purview sensitivity label) and docProps/custom.xml. All were
dropped on save, so a labelled corporate deck came back unlabelled. They
are now kept verbatim with their content types, their own relationships
(itemProps) and the root or presentation relationship linking them. Kept
parts never replace a part the writer makes.

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/file-format-parity.md

@@ -83,7 +83,8 @@ the round trip even though DeckCraft doesn't model it.
 | Comments: legacy | yes | yes | |
 | Comments: modern threaded (`p188`) | yes | no (written as legacy) | Replies and resolved state lost (#70) |
 | Unknown slide-level elements (`graphicFrame` OLE, extensions, `mc:AlternateContent`) | kept | kept | Rendered as a labelled placeholder with the preview picture when present |
-| Tags, custom XML, VBA | no / no / no | | |
+| Custom XML data, sensitivity label (`docMetadata/LabelInfo.xml`), custom document properties | kept verbatim | yes | SharePoint metadata and the label survive a save; `custom_xml_label_and_custom_properties_survive_save` |
+| Tags, VBA | no / no | | |
 | Embedded fonts | no | no | |
 | Encryption (password to open) | no | no | |
 
@@ -99,6 +100,7 @@ the round trip even though DeckCraft doesn't model it.
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-13 | minor | Custom XML parts, the sensitivity label and custom properties kept on save |
 | 2026-10-12 | minor | EMF/WMF text and pattern brushes drawn; OLE (VML) and ActiveX previews read |
 | 2026-10-12 | minor | Text warps rendered; their adjust values read and written |
 | 2026-10-12 | trivial | Groups with a small legacy child coordinate space draw outlines, dashes, arrowheads and ink at their own width (#233) |