htmlToMarkdown's <a> case unconditionally wrapped its inner text as
[inner](href). In hybrid source mode the <a> already contains its
`[` / `](url)` marker spans as children, so `inner` was already the full
`[label](url)` markdown — wrapping again doubled it into
`[[label](url)](url)`, compounding on every edit (e.g. editing a comment
or post link).

Add a hasSourceMarkerChild guard to the <a> case (mirroring the inline
serializer's _hasMarkerChildren check): return inner as-is when the link
already carries source markers, only synthesizing `[...](url)` for a bare
<a> (rendered/visual mode, Earmark, createLink). Companion to the 0.2.20
builder-side fix for `****bold****` compounding.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>