feat(tabs): draw lineage lines as one tree for the selected tab's family

Lineage lines used to draw one glowing dashed bezier per parent/child pair,
hanging below the tab strip, for every family at once. With a parent on row 3
of a wrapped strip and ten children below it, the curves crossed every lower
row's tab names and ran through the terminal text.

- One rounded orthogonal tree per spawning tab: every route starts at the
  parent, so siblings share a trunk. 1.5px, solid, 1px dark outline; only a
  working child's branch is dashed.
- Routes run only through the gaps between tab rows, joined by a spine left of
  every row (computeLineageTree/computeLineageRows in constants.js), so they
  never cross a tab or reach the terminal. The vertical rail gets the same tree
  on its existing left track.
- Only the selected tab's family is drawn (what it spawned, plus its parent and
  siblings). Selection redraws, and the strip's size transitionend redraws once
  more, since the active tab widens for ~150ms after the selection redraw.
- The routing room is reserved by .session-tabs.lineage-tree, keyed on whether
  any lineage exists, never on the selection, so a tab switch never resizes the
  header or the PTY.
- Colours stay per spawning tab, now claimed in strip order for every family so
  selection order never decides who gets the skin blue.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/wiki/The-Dashboard.md

@@ -84,13 +84,17 @@ title is derived locally from the prompt's first sentence; no text leaves the ma
 On phones the strip scrolls horizontally instead of wrapping, and the active tab is always
 scrolled into view. It is not reordered to the front, so the `Alt+N` numbering stays stable.
 
-### Lineage arcs
+### Lineage lines
 
-When one session spawns another (an agent starting a worker through the API), Codeman draws
-a coloured arc under the strip connecting parent to child, with one colour per child. It is
-how a fan-out of eight workers stays readable.
+When one session spawns another (an agent starting a worker through the API), selecting
+either tab draws its family: lines from the parent to each child, in the parent's colour,
+routed through the gaps between tab rows so they never cover a tab or the terminal. A dashed
+branch means that child is working. It is how a fan-out of eight workers stays readable.
 
-Desktop only, and on by default. Turn it off in **App Settings → Appearance**. Arcs are
+While any tab has spawned another, the strip keeps a little extra room between rows for the
+lines, so switching tabs never changes the header height.
+
+Desktop only, and on by default. Turn it off in **App Settings → Appearance**. Lines are
 skipped for tabs scrolled out of the strip.
 
 ## Header controls