BlockRenderer typedef

Function type for rendering block-level nodes.

This is the public callback contract and is intentionally kept single-argument: a plain (nodes) => widget function is assignable here, so existing callbacks keep working. It deliberately carries no context parameter so that changes to the context model never become a source-breaking change for callers that only render children. Use ContextualBlockRenderer (via MarkdownRenderContext.contextualBlockRenderer) when a builder needs to override the render context.