Finish flattened content helpers render step

[Copilot]

This picks up the unfinished work from the abandoned flattened-content helper PR and carries it to a reviewable state. It adds the helper generation step, wires it into the public renderer config, and closes the remaining gaps in naming and generated output coverage.

• What changes

  • Adds ContentHelpersRenderStep to generate accessor methods for flattened Vec<...Content> struct content.
  • Exposes the step through RenderStep so it can be enabled via normal renderer configuration.
  • Adds feature coverage for generated helpers, including keyword-derived element names.

• Renderer integration

  • Registers the new step in renderer/config plumbing.
  • Exports the step from the renderer module tree.
  • Keeps the implementation scoped to flattened dynamic-list content backed by an inline enum.

• Generated API

  • Emits read and mutable helper accessors for single-occurrence content variants.
  • Preserves valid Rust naming for keyword-derived identifiers when generating *_mut helpers.

impl FooType {
    pub fn bar(&self) -> Option<&String> { ... }
    pub fn bar_mut(&mut self) -> Option<&mut String> { ... }

    pub fn type_(&self) -> Option<&String> { ... }
    pub fn type_mut(&mut self) -> Option<&mut String> { ... }
}

• Coverage
  • Adds a focused feature fixture for flattened content helpers.
  • Adds a regression case for keyword-based names such as type / match.

Closes #271