Implement `<SelectableLabel>` tag (egui::SelectableLabel)

[ZhukMax]

Provide a <SelectableLabel> tag for single-item selection UX or list-like UIs. Supports a controlled selected={bool} binding and click handling.

• Common building block for custom lists, nav rails, and option rows.
• Pairs well with ScrollArea and columns/rows to build list UIs.

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Scope

• Tag: <SelectableLabel>
• Attributes: selected={bool}, onClick={|| ...}, enabled, wrap, tooltip.
• Children: text/inline content (same as <Label>).

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Proposed API (sketch)

efx!(ui, r#"
<SelectableLabel selected={tab == "home"} onClick={|| tab = "home"}>Home</SelectableLabel>
<SelectableLabel selected={tab == "settings"} onClick={|| tab = "settings"}>Settings</SelectableLabel>
"#);

// Mapping
// ui.add(egui::SelectableLabel::new(selected, "Text"));

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Tasks

• AST + codegen for <SelectableLabel> with text/children support.
• Controlled selected binding + onClick event hookup.
• Attribute adapters: enabled, wrap, tooltip.
• Examples: simple tab-like switcher; list with multiple items.
• trybuild UI tests for missing/invalid attrs.
• Ensure examples build for wasm32-unknown-unknown.