Package-level declarations

Native-where-possible, HTML-fallback-where-not. The result reads like hand-written docs for the 90% that maps, and only the rare orphan constructs land in the (styled-by-shim) fallback branch.

The policy layer: which MyST nodes have a native Starlight/MDX equivalent, and how MyST's ~10 admonition kinds collapse onto Starlight's 4 aside types. Everything here is a deliberate choice, which is exactly why it lives in code you own rather than in a generic library.