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>

parity/acrobat-features.toml

@@ -4689,6 +4689,18 @@ tools = ["form_add_field"]
 evidence = ["crates/forms/src/tests.rs::every_field_type_can_be_added_named_and_drawn", "crates/automation/tests/automation.rs::preparing_a_form_through_tools"]
 notes = "new group Group1… or join the selected group with the next ChoiceN"
 
+[[feature]]
+id = "form.check-box-style"
+title = "Check box and radio button styles"
+area = "F"
+tier = "P1"
+milestone = "M6"
+status = "shipped"
+commands = ["form.field.properties"]
+tools = ["form_set_props", "form_fields"]
+evidence = ["crates/forms/src/tests.rs::check_box_styles_change_the_mark", "crates/ui-egui/tests/prepare.rs::options_tab_sets_the_check_box_style", "crates/automation/tests/automation.rs::preparing_a_form_through_tools"]
+notes = "Field Properties ▸ Options: check, circle, cross, diamond, square, star (/MK /CA, Acrobat's codes), drawn as paths; defaults check (check boxes) and circle (radio buttons)"
+
 [[feature]]
 id = "form.add-dropdown"
 title = "Add drop-down lists"