M4: Document Properties has a Custom tab for custom properties (#4387)

* M4: Document Properties has a Custom tab for custom properties

Document Properties has a new Custom tab between Fonts and Advanced. It
lists the document's custom properties, and Name and Value fields with
Add, Update and Delete edit the list. Clicking a listed property fills
the fields with it. OK writes the changes through SetInfo in the
existing "Change document properties" step, so one undo reverts them
with the other tabs' changes.

Add is off for an empty value, a name already listed, a name that
check_custom_key refuses (its message is the button's tooltip), and a
name the file already uses for an entry the list leaves out (not text,
only padding, or past the 256 listed). A property already in the file
under a refused name or a name that is not UTF-8 is listed, and Add,
Update and Delete stay off for it, so OK cannot fail on it or write a
different key.

Clearing an /Info entry now removes it when /Info is a dictionary
written directly in the trailer. set_info_entry returned without a
change there, so a delete in the Custom tab, a cleared Description field
and a doc_set_info clear left the entry in the file. It now writes a
copy without the entry as a new indirect /Info.

The engine re-exports check_custom_key and adds Document::has_info
(organize::has_info). set_option("dialog", "custom") opens the tab.

* Contributors: add dotCooCoo (Robert Lee)

* M4: Add in the Custom tab stops at the 256 properties the tab lists

Document Properties > Custom lists at most 256 properties, so a property
added past that was saved but left out of the list when the dialog opened
again. Add is now off once the tab's list or the file's own list holds 256;
a new name goes after the file's entries, so a file with more than 256 keeps
Add off after a Delete as well.

docs/pdf-spec-parity.md

@@ -1,6 +1,6 @@
 # PDF specification and standards parity
 
-> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-14 · **Change:** minor (§12.4.4: page transitions and display times read) · **Target:** Adobe Acrobat Pro (Acrobat DC, continuous track 26.002.21931, macOS)
+> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-24 · **Change:** minor (§14.3: custom properties edited in Document Properties) · **Target:** Adobe Acrobat Pro (Acrobat DC, continuous track 26.002.21931, macOS)
 
 How much of ISO 32000-2 (PDF 2.0) and the PDF subset standards PdfCraft implements, set against
 what Acrobat Pro implements. This is PdfCraft's equivalent of a "geometry" or "codec" checklist:
@@ -44,7 +44,7 @@ are borrowed; PDF 2.0 additions and most subset standards are missing. **150–2
 | §12.9 | Measurement and geospatial | full | 2D measuring with viewports and calibration; no geospatial | `comment.geospatial` planned |
 | §12.10 | Document requirements | yes | none | |
 | §13 | Multimedia: sound, movies, screen, rich media, 3D (U3D, PRC) | yes | preserved untouched; not played or shown | `misc.rich-media-*`, `misc.3d-view` planned (P3) |
-| §14.3 | Metadata: info dictionary, XMP | full, XMP editor | info dictionary edited, with its XMP counterparts (title, author, subject, keywords, creator tool, producer) kept in step; XMP written for PDF/A; no XMP editor or custom properties | `organize.xmp-editor` planned |
+| §14.3 | Metadata: info dictionary, XMP | full, XMP editor | info dictionary edited, with its XMP counterparts (title, author, subject, keywords, creator tool, producer) kept in step; XMP written for PDF/A; custom properties listed, added, changed and deleted (Document Properties ▸ Custom, `doc_info`, `doc_set_info`); no XMP editor | `organize.custom-properties` partial, `organize.xmp-editor` planned |
 | §14.6–14.8 | Marked content, logical structure, tagged PDF | full, with editing tools | read (checker, bookmarks from structure, redaction of tags); no tag editing or autotag | gap 11 |
 | §14.9 | Accessibility support: alt text, ActualText, language | full | alt text figure by figure, language, title | `a11y.alt-text-workflow` |
 | §14.10 | Web capture | yes | none | |