Implement <Checkbox> tag (egui::Checkbox)

[ZhukMax]

Add a declarative <Checkbox> widget with a controlled boolean binding and optional change handler. Supports inline text (children) or a text="..." attribute, plus common UX knobs like enabled, wrap, and tooltip.

• Covers a core form/setting control with an idiomatic, template-first API.
• Aligns with EFx’s controlled-input model (value comes from state; emits onChange).
• Reduces imperative glue for simple preference panes and dialogs.

---

Scope

• Tag: <Checkbox>
• Required: checked={bool_expr}
• Optional attributes:
  text="...", enabled=bool, wrap=bool, tooltip="...", id="..." (for stable identity)
• Events: onChange={|checked: bool| ...} (fires once when the value toggles)
• Content: either text="..." or inline text children (mutually exclusive; diagnostics on conflict)

Non-goals (here): tri-state/indeterminate mode, custom icons, per-state colors.

---

Proposed API (sketch)

Template

// With children text
efx!(ui, r#"
<Checkbox checked={prefs.notifications} onChange={|v| prefs.notifications = v} wrap="true">
  Enable notifications
</Checkbox>
"#);

// With text attribute
efx!(ui, r#"
<Checkbox checked={prefs.autosave} text="Autosave files" tooltip="Saves every 5 minutes"/>
"#);

Conceptual mapping to egui

let mut val = /* bound bool */;
let mut widget = egui::Checkbox::new(&mut val, text_str); // text_str from children or text=""
widget = widget.wrap(wrap);

let resp = if enabled {
    ui.add(widget)
} else {
    ui.add_enabled(false, widget)
};

if let Some(tip) = tooltip { resp.on_hover_text(tip); }
if resp.changed() {
    // emit onChange(val)
}

---

Tasks

• AST & parsing

  • Add <Checkbox> node with checked (bool) and optional text, enabled, wrap, tooltip, id.
  • Allow exactly one text source: children or text=...; error on both.

• Codegen

  • Bind checked={expr} by value-copy to a local, pass &mut to egui::Checkbox, then write back if changed.
  • Emit onChange(bool) when Response::changed() is true.
  • Support enabled=false via ui.add_enabled(false, ...).
  • Apply wrap flag and tooltip on Response.
  • Honor optional id using egui::Id if provided (stable identity).

• Diagnostics

  • Missing checked or non-boolean type → clear compile error with expected type.
  • Both children text and text= present → actionable error (choose one).
  • Unknown/unsupported attributes → list allowed attrs for <Checkbox>.

• Examples & docs

  • Minimal preferences pane with two checkboxes (children vs text=).
  • Disabled checkbox with tooltip; long text demonstrating wrap.
  • Documentation snippet in docs/cookbook.md and docs/tags.md.

• Tests

  • trybuild UI tests: missing checked; wrong type; dual text sources; unknown attrs.
  • Unit test: toggling flows updates and fires onChange exactly once.
  • Ensure examples build for wasm32-unknown-unknown (no runtime launch).