Expose Markdown headings and migrate to native Moon CI

[meoyawn]

Markdown rendering now returns heading metadata alongside HTML, so consumers can build an accessible table of contents using the same IDs as the rendered article. IDs are unique within a document, preserve existing IDs, and handle duplicate, Unicode, and empty headings. Content collection loaders expose this metadata as well.

Replace Taskfile with reusable Moon tasks and a standalone workflow following Moon’s CI guide: full history, frozen dependencies, plain moon ci, explicit inputs/outputs, and native reports. The same moon.yml works inside Listenbox’s submodule graph. Publishing remains a manual task.

Validation: standalone native Moon CI passed all six tasks from a fresh clone, including 37 unit tests, six browser tests, lint, and both TypeScript checks. Listenbox’s guide integration passed 53 marketing browser tests and a build starting with no compiled Solid Static package. Package lint uses its own configuration in both workspaces.