heading slug

GitHub-style fragment id for a heading in compiled Markdown (the part after # in [label](#slug)). Parent concept: refs.

Merlon Docs computes slugs from final heading text after guides are stitched and demoted, with the same rules GitHub uses for README anchors (via github-slugger). Duplicate titles in one document get -1, -2 suffixes. Authors should not invent fragments from shard-only titles; cross-links must match the compiled slug, and merlon check fails when they do not.