Forms: say so when a document is an XFA form (#60)

XFA forms aren't read yet, and opening one gave no hint: a dynamic XFA form
showed only its "Please wait…" placeholder page, and a static one looked like
an ordinary form though Acrobat shows its XFA data in preference to the
fields that get filled.

- render: DocInfo.xfa (Static / Dynamic) from /AcroForm /XFA; dynamic when
  /NeedsRendering is true or there are no fields. Exported for doc_info
  (automation and CLI) as "xfa".
- ui: the notice bar explains each case (above the form-fields notice); a
  static form keeps its Highlight fields button.
- Tests: tests/xfa.rs (dynamic with and without /NeedsRendering, static,
  ordinary forms, a malformed /XFA). Parity notes for form.xfa-static and
  form.xfa-dynamic (still planned: this explains XFA, it doesn't support it).

Reported by @Sudachi-dev.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

parity/acrobat-features.toml

@@ -5107,7 +5107,7 @@ status = "planned"
 commands = []
 tools = []
 evidence = []
-notes = ""
+notes = "not read yet; detected on open (DocInfo.xfa, doc_info \"xfa\") and a notice says the fields can be filled but Acrobat may show the XFA values instead (#60)"
 
 [[feature]]
 id = "form.xfa-dynamic"
@@ -5119,7 +5119,7 @@ status = "planned"
 commands = []
 tools = []
 evidence = []
-notes = ""
+notes = "not rendered yet; detected on open and a notice says the page shown is the file's placeholder (#60)"
 
 [[feature]]
 id = "form.xfa-formcalc"