mapDocument re-ran the alphabetical min() scan over the letter's tag set to
look up its already-resolved root, duplicating the work resolveLetterRootTags
had just done and leaving two independent definitions of "primary tag" that
could silently diverge. Key the resolved-root map by document id and compute
the primary tag exactly once per letter; drop the redundant resolvePrimaryRoot
helper.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>