The incremental tree editor diverged from buildTree on realistic filename distributions. OldTreeCursor.appendPrefixAtHeight read the retained native-prefix list without consuming it, so one descriptor could be spliced into two different output levels: buildUnpublished placed the height-1 prefixes into the leaves, and mergeRootPrefixes then prepended the identical descriptor again before finishRootLevel. The emitted interior node carried the same S key twice, which fails artifact.parse's ascending-S guard, so an import aborted partway through a real history with a SyntaxError raised by our own grammar.

finishCheckpoint already guarded its own splice with a height test. That guard was an ad-hoc workaround for this same double-append, and mergeRootPrefixes never had one. The splice now consumes what it emits, so a retained descriptor lands in exactly one level and the guard becomes unnecessary rather than duplicated.

Found by importing marktext, not by the suite. Every name corpus here was uniform, and a duplicated prefix descriptor requires names that share prefixes at differing depths, so 55 tests could pass over a defect that breaks real repositories. The new seeded differential sweep generates four corpora of 136 to 240 paths with mixed depth, shared directories, dotfiles and 1 to 43 byte basenames, over every supported k with twelve batches each, and asserts it reached a tree at least three levels deep so it cannot quietly degrade into testing flat roots. It diverges in 20 of 452 batches against the unfixed editor, every divergence unparseable.

The decoded-root validation memo lands alongside it: repeated lookupTree validation against an unchanged root is answered once instead of rebuilt, which takes this file's own 56 tests from 1701 to 450 seconds over identical assertions. check-in: 4a95e436af user: dmestas tags: sub-manifest