Add extensible LLMS Markdown serializers for custom MDX/JSX components

[LadyBluenotes]

Summary

SolidBase currently generates LLMS/document-markdown output by converting route content into Markdown-oriented text, but custom MDX/JSX components cannot be serialized with guaranteed semantics.

We can normalize built-in wrappers, but user-defined JSX components are still ambiguous without an explicit conversion.

Goal

Add an extension that lets users define how custom MDX/JSX components should be converted into plain Markdown for LLMS export.

Proposed Direction

Add an LLMS/document-markdown serializer registry, for example under config:

llms: {
  componentSerializers: {
    Callout(node, ctx) {
      // return mdast nodes or markdown text
    },
    ApiTable(node, ctx) {
      // custom conversion
    },
  },
  unknownComponentBehavior: "flatten" | "warn" | "error" | "drop",
}

Requirements

• guarantee plain Markdown output for LLMS artifacts
• allow user-defined serializers for custom JSX components
• provide a configurable fallback for unknown components
• support a strict mode that fails generation on unknown JSX
• keep output deterministic and testable

Acceptance Criteria

• custom JSX components can be serialized through config without patching library internals
• unknown JSX does not silently leak raw MDX into .md LLMS output
• strict mode can fail generation when unsupported JSX is encountered
• docs explain serializer behavior, fallback modes, and strict mode