The container-row tree-prefix used text box-drawing glyphs (├ └ │)
positioned with `top: 0.6em` — a single text-line tall. Once rows
grew past one line (5em square icon + multi-line body), the `│`
columns of consecutive siblings no longer touched, leaving visible
breaks in the tree.
Replace the text-glyph string with structured DOM: one `.tree-lane`
per depth column. Continuation lanes (`.lane-line`) paint a 1px
border-left spanning the full row height + the `.containers` gap
below, so adjacent siblings' bars visually merge into one unbroken
vertical. The row's own joint lane is `├` (branch — bar continues
below) or `└` (last — bar stops at icon midline), with a horizontal
stub at 3.1em (row padding-top + icon half-height) reaching to the
icon edge.
Joint y / stub width are derived from the 5em icon + 0.6em row
padding-top + 0.8em row padding-left so they meet the icon cleanly.
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