Fix: xs:element default value ignored for simpleContent extension types

[Copilot]

When an xs:element carries default="false" and wraps an inline complex type with xs:simpleContent (e.g. extending xs:boolean), a self-closing element like <Bar Baz="xxxx"/> fails deserialization with UnknownOrInvalidValue("") — the default is never applied.

Root cause: Event::Empty (self-closing XML element) was passed directly to ContentDeserializer::init, which tried to parse an empty byte slice as a boolean. The default attribute from xs:element was never threaded through the pipeline to the generated deserializer.

Changes

• ComplexMeta (models/meta/complex.rs): Add default: Option<String> field; include in TypeEq so types with different defaults remain distinct.
• Interpreter (pipeline/interpreter/state/generate_types/variant_processor.rs): Propagate xs:element.default → ComplexMeta.default when building inline complex types.
• Generator (pipeline/generator/data/complex.rs): Convert the default string to a ValueRendererBox; store it in ComplexDataContent.default_value. Thread a default: Option<&str> parameter through new(), new_simple(), and all call sites.
• ComplexDataContent (models/data/complex.rs): Add default_value: Option<ValueRendererBox> field.
• Defaults renderer (pipeline/renderer/steps/defaults.rs): Emit a pub fn default_content() -> T { … } associated function when default_value is set.
• Deserializer renderer (pipeline/renderer/steps/quick_xml/deserialize.rs): In render_deserializer_fn_next_content_simple, intercept Event::Empty before calling ContentDeserializer — if a default exists, call default_content() and return immediately.

Generated output example

impl BarElementType {
    #[must_use]
    pub fn default_content() -> bool {
        false
    }
}

// In the generated deserializer:
S::Init__ => {
    if matches!(&event, Event::Empty(_)) {
        self.store_content(super::BarElementType::default_content())?;
        let data = self.finish(helper)?;
        return Ok(DeserializerOutput { artifact: DeserializerArtifact::Data(data), .. });
    }
    let output = ContentDeserializer::init(helper, event)?;
    self.handle_content(helper, output)
}

• Fixes Element with extension base type "xs:boolean" ignores default value #284