fix(tabs): wrap case clusters box by box and never route a lineage line through a tab

With tabArrangement 'case' at 1440px (nine tabs in four cases), the lineage
routes ran horizontally through the middle of tabs, cluster labels and box
borders. #538's clusters are `max-width: 100%` boxes that may shrink, so in
the one-line strip every box squeezed and wrapped inside itself (a one-tab
case's swatch alone on a line above its tab). The strip then never
overflowed, so updateTabOverflowMode() never wrapped it, and #544's routing
room (`.lineage-tree.tabs-auto-wrap`: row gap and spine channel) never
applied. computeLineageRows grouped the squeezed tabs by top alone into rows
that overlap (10-40, 20-50, 42-74), and gapUnder put the first row's gap at
y 30, inside the tabs below it.

- On the desktop header strip a cluster keeps its width, so clusters that do
  not fit overflow and the strip wraps box by box; the boxes' own gaps read
  --lineage-row-gap, so the routes run between box rows and through the
  spine channel. A case wider than the whole strip still wraps inside its
  box, and now wraps the strip too (`_tabClustersWrapInside`, the new
  `innerWrap` input of shouldAutoWrapTabs), so its inner rows get the gap.
- computeLineageRows never returns overlapping rows: tabs whose spans
  overlap are one row. gapUnder returns null when there is no real gap, and
  a route with no gap is not drawn. A final pass drops any route with a
  segment inside a tab, so no arrangement can draw a line through a tab
  (a layout without the room loses lines instead). Pinned with the squeezed
  rects measured live.

At 1920px, where the clusters fit on one line, nothing changes.

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

src/web/public/styles.css

@@ -20891,6 +20891,21 @@ html[data-session-list='sidebar'][data-sidebar='collapsed'] .session-sidebar .ta
   padding-left: 3px;
 }
 
+/* Desktop: a cluster keeps its width, so clusters that do not fit on one line
+   overflow the strip and updateTabOverflowMode() wraps it box by box. Allowed
+   to shrink, every box squeezed and wrapped inside itself instead (a one-tab
+   case's swatch alone on a line above its tab), the strip never overflowed,
+   and the lineage routing room, which needs a wrapped strip, never applied:
+   the routes ran through tabs, labels and box borders. Only a case wider than
+   the whole strip still wraps inside its box (max-width: 100%), and the strip
+   wraps around it too (_tabClustersWrapInside). The narrower screens keep
+   their scrolling strip. */
+@media (min-width: 768px) {
+  .session-tabs-host > .session-tabs.tabs-clusters > .tab-cluster {
+    flex-shrink: 0;
+  }
+}
+
 /* Side rail and sidebar: every case is a section with its label on top
    (vertical space is cheap there, so a one-tab case is labelled too). The
    label goes first by `order`, because a sorted rail orders the rows inside