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/app.js

@@ -6359,12 +6359,29 @@ class CodemanApp {
           tabCount: this.sessions.size,
           scrollWidth: container.scrollWidth,
           clientWidth: container.clientWidth,
+          innerWrap: container.classList.contains('tabs-clusters') && this._tabClustersWrapInside(container),
         })
       : container.scrollWidth > container.clientWidth + 1;
 
     container.classList.toggle('tabs-auto-wrap', shouldWrap);
   }
 
+  /**
+   * True when a case cluster in the header strip wraps inside its own box: a case
+   * wider than the whole strip (styles.css caps a box at the strip's width). The
+   * strip then has rows although it never overflows, so it must still wrap, or
+   * the lineage routing room (`.lineage-tree.tabs-auto-wrap`) is never reserved
+   * and the routes have no gap between the box's rows to run in. Read right
+   * after the overflow measure, so layout is already clean.
+   */
+  _tabClustersWrapInside(container) {
+    for (const box of container.querySelectorAll(':scope > .tab-cluster')) {
+      const tabs = box.querySelectorAll(':scope > .session-tab');
+      if (tabs.length > 1 && tabs[tabs.length - 1].offsetTop > tabs[0].offsetTop + 4) return true;
+    }
+    return false;
+  }
+
   // Middle-click closes a tab, mirroring browser tab strips. Session tabs go
   // through requestCloseSession (the same confirm modal as the x button), web
   // tabs through closeWebviewTab (same as theirs). Delegated on the container: