The backend dropped the agent hierarchy's parent field, so every
container is a root and buildAgentTree/treePrefixDom could only ever
produce a single-level flat list — the .tree-prefix CSS lane rules
already matched nothing. Replaced with sortedContainerRows, a plain
alphabetical sort, and dropped the now-dead .tree-prefix/.tree-lane/
data-depth CSS and the depth/isLast/ancestorIsLast fields from the row
fingerprint and buildContainerLi's signature. No visible behavior
change - the rendered list was already flat, just via dead machinery.
Docs updated to describe the simpler implementation directly instead
of narrating the removal (kept the heading name since swarm.js still
points a comment at it).
6b5c8cc51a