fix(ui): bind lineage line colours to the spawning tab

Lineage arcs were coloured per child, so one tab's own workers each got a
different colour, which is the distinction the colours exist to make. The colour
is now keyed on the parent: every arc leaving one tab is the same colour however
many workers it spawns, so the strip reads as "these five came from w1, those
two came from w2". A child that spawns in turn is a parent in its own right and
gets its own colour for the arcs below it, so a chain changes colour at each
generation while each generation's fan-out stays uniform.

The new tests drive the real _appendLineageConnectionLines() and assert the
painted custom property, because testing the colour function alone passes just
as happily with the child id passed back in.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/web/public/constants.js

@@ -243,7 +243,9 @@ const LINEAGE_DIP_MAX_PX = 64;
 // apart bled into one thick band instead of reading as three separate lines.
 const LINEAGE_SIBLING_STEP_PX = 8;
 const LINEAGE_STRIP_TOLERANCE_PX = 4;
-// Lineage palette, assigned per CHILD in first-seen order and cycled (session-lineage.js).
+// Lineage palette, assigned per SPAWNING TAB in first-seen order and cycled
+// (session-lineage.js). Every arc leaving one tab shares its colour however many
+// workers it spawns; a child that spawns in turn gets its own for the arcs below it.
 // The empty FIRST entry means "no override": the CSS then falls back to --session-blue,
 // which every skin block tunes for its own background, so a lone arc keeps the
 // skin-aware blue that shipped in 1.18.2. The fixed entries are deliberately vivid