Nice write-up, Oliver. The "sparse index with delayed writers" framing really clicked for me. It also explains why the headings work so well mechanically: Git's three-way merge conflicts on overlapping or even adjacent changes, and each heading guarantees at least one untouched line between write points.
One question: within a section, are new entries sorted, or inserted directly under the heading? With the latter, any two same-letter additions would still collide. That matters because first letters aren't evenly spread, especially for software topics (api, auth, cache, ci, config...), so C and S will probably run hot.
Also, since agents love to "tidy" files (removing empty headings, re-sorting, turning lists into tables), a small CI check that all 36 headings exist and each entry sits under the right one might be what keeps the convention alive long term.
Would love to see numbers on this at some point. git merge-tree --write-tree makes it pretty easy to simulate append vs. sorted vs. sectioned.