AcroForm field access for documents and editors.

fields/1 reads from either source — a read-only PdfElixide.Document or a mutable PdfElixide.Editor (source/0) — so inspecting a form needs no editor. Writing does: set_value/3 takes an Editor only, since a document cannot be changed.

# Inspect, read-only.
doc = PdfElixide.Document.open!("form.pdf")
fields = PdfElixide.Form.fields!(doc)

# Fill and persist.
editor = PdfElixide.Editor.open!("form.pdf")
:ok = PdfElixide.Form.set_value(editor, "full_name", {:text, "Jane Doe"})
:ok = PdfElixide.Form.set_value(editor, "subscribe", {:boolean, true})
:ok = PdfElixide.Editor.save(editor, "filled.pdf")

Fields come back as PdfElixide.Form.Field structs, and the tagged tuple a field's :value carries is the same shape set_value/3 accepts, so a value read from one form can be written straight into another.

Fields are addressed by name, and only an existing field can be set — there is no way to add one. A name that is not in the form is {:error, %PdfElixide.Error{}}.

Which lock fields/1 takes follows its source: a shared read on a PdfElixide.Document, and the editor's exclusive lock on a PdfElixide.Editor, exactly as set_value/3 takes. So concurrent form work on one editor serializes even when it only reads; see the Concurrency guide.