M6: the forms README says which metadata declares PDF/A

declares_pdfa reads pdfaid:part from XMP metadata that decodes within
8 MiB, as preflight's Standards panel reads it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AmsdeUCdTaD72jkDw2pJKY

crates/forms/README.md

@@ -50,7 +50,8 @@ let cells: Vec<table::Cell> = grid.cells_for(names)?;      // "Table1_1", "Table
   keeps it: `/V` is stored as typed, the appearance draws `?` for those characters, the widget is
   marked with the first one (`pdfcraft_fonts::UNDRAWN_MARK`, cleared once all of it can be drawn)
   and the form sets `/NeedAppearances`, so other viewers draw the field from its value with fonts
-  that have them. Not in a document that declares PDF/A, which forbids that flag (`declares_pdfa`).
+  that have them. Not in a document that declares PDF/A, which forbids that flag (`declares_pdfa`:
+  `pdfaid:part` in XMP metadata that decodes within 8 MiB, as preflight's Standards panel reads it).
   `undrawn_fields` lists the marked fields; the engine reports each edit that draws one, and
   flattening refuses to make that `?` permanent.
 - Displaying a form that leaves its appearances to the viewer (`/NeedAppearances`, or a text or