Forms: check box and radio button styles (#94)

Field Properties > Options > Check Box Style (Button Style for radio
buttons): Check, Circle, Cross, Diamond, Square or Star, as Acrobat
offers. The choice is stored in each widget's /MK /CA with Acrobat's
ZapfDingbats codes (4 l 8 u n H), so other viewers draw the same mark,
and PrintCraft draws it as paths, needing no symbol font. Styles set
by other apps are read back too; the defaults stay a check for check
boxes and a circle for radio buttons.

- forms: CheckStyle, check_style(), FieldProps::check_style; set_props
  writes /MK /CA (resolving an indirect /MK so its colours survive) and
  redraws; text and choice fields refuse a style.
- engine: Document::field_check_style.
- automation: form_set_props takes check_style, form_fields reports it.

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

crates/engine/src/lib.rs

@@ -32,8 +32,8 @@ pub use printcraft_edit::{
     Added, AddedImage, AddedText, Align as TextAlign, Background, Content as AddedContent, Family as FontFamily, HeaderFooter, MarkKind, Watermark,
 };
 pub use printcraft_forms::{
-    BorderStyle, Field as FormField, FieldAction, FieldFont, FieldKind as FormFieldKind, FieldProps, FieldValue, Look as FieldLook, NewField,
-    TabOrder, Trigger as FieldTrigger, Widget as FormWidget, af as form_scripts, flags as field_flags,
+    BorderStyle, CheckStyle, Field as FormField, FieldAction, FieldFont, FieldKind as FormFieldKind, FieldProps, FieldValue, Look as FieldLook,
+    NewField, TabOrder, Trigger as FieldTrigger, Widget as FormWidget, af as form_scripts, flags as field_flags,
 };
 
 pub use printcraft_a11y as a11y;
@@ -386,6 +386,13 @@ impl Document {
         Some(printcraft_forms::look(&e.cos, f))
     }
 
+    /// A check box's or radio button's mark (Field Properties ▸ Options), `None` for other fields.
+    pub fn field_check_style(&self, name: &str) -> Option<CheckStyle> {
+        let e = self.editor.as_ref()?;
+        let f = self.form.iter().find(|f| f.name == name)?;
+        matches!(f.kind, FormFieldKind::CheckBox | FormFieldKind::Radio).then(|| printcraft_forms::check_style(&e.cos, f))
+    }
+
     /// Remove Hidden Information: what each category would remove.
     pub fn hidden_info(&self) -> Vec<(Hidden, usize)> {
         self.editor.as_ref().map(|e| printcraft_redact::sanitize::scan(&e.cos)).unwrap_or_default()