fix(ui): make session lineage lines read as arcs, not straight threads

The lines that join a tab to the workers its codeman skill spawned were
drawn with numbers tuned against two tabs sitting side by side, and they
degraded in exactly the two situations the feature is actually used in.

1. A spawned worker is appended to the END of the strip, so the real span
   between a lead and its worker is 800-1500px. With the dip clamped at
   44px that is a 33px sag: the arc reads as a straight line drawn across
   the terminal instead of a bracket hanging under the strip. The dip now
   grows at 0.085/px and clamps at 104.

2. When the desktop strip wraps (tabs-two-rows / tabs-auto-wrap), a parent
   on row 1 and its child on row 2 are ~14px apart, and the cross-row
   branch drew parent-bottom to child-TOP: a flat line hidden inside the
   row gap, with siblings overprinting each other. Both ends now anchor on
   the tab BOTTOM with the control points below the LOWER row, so a wrapped
   pair gets the same bracket a flat strip gets. That deletes the branch:
   one shape covers both.

Visibility, at 1:1 rather than in a zoomed mockup: 2 -> 2.5px stroke,
4 4 -> 5 5 dashes (lineage-flow moves with them, -16 -> -20), opacity
.55 -> .72, and a second wider glow so the contrast comes from the halo
rather than from more weight, keeping the line under the subagent lines'
3px. A working child is bright (.95) outside the reduced-motion block, so
turning motion off no longer also dims every worker's arc. Sibling nesting
6 -> 8px and the direction dot 3 -> 3.5px to match the heavier stroke.

Verified at 1:1 in a harness driving the real styles.css and the real
computeLineagePath over three layouts (adjacent workers, workers at the
far end of a full strip, wrapped two-row strip) on a dark and a light
skin. test/session-lineage-lines.test.ts pins both regressions.

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

test/session-lineage-lines.test.ts

@@ -77,25 +77,47 @@ describe('lineage line geometry', () => {
     expect(first.d).not.toBe(second.d);
   });
 
-  it('switches to a vertical bezier when the strip has wrapped to two rows', () => {
+  it('keeps bending at strip-wide spans instead of flattening into a straight line', () => {
     const helper = loadLineageHelper();
+    // A worker the agent skill starts is appended to the END of the strip, so this
+    // is the span the feature is actually used at. The first shipped clamp (44px)
+    // turned it into a flat thread across the terminal.
+    const wide = helper.computePath({ parent: tab(0), child: tab(1300), strip: { ...STRIP, width: 1500 } })!;
+    const near = helper.computePath({ parent: tab(0), child: tab(140), strip: STRIP })!;
+
+    const wideDip = controlYs(wide.d)[0] - 34;
+    const nearDip = controlYs(near.d)[0] - 34;
+    expect(wideDip).toBeGreaterThan(nearDip * 2);
+    expect(wideDip).toBeGreaterThanOrEqual(80);
+  });
+
+  it('brackets a wrapped pair BELOW the lower row rather than inside the row gap', () => {
+    const helper = loadLineageHelper();
+    // The reported bug: with the desktop strip wrapped, a parent on row 1 (bottom 34)
+    // and its child on row 2 (top 48) are 14px apart, and a parent-bottom → child-TOP
+    // bezier had 14px to bend in, so it drew a flat line hidden in the gap, three
+    // siblings overprinting each other. Both ends now anchor on the tab BOTTOM and the
+    // curve hangs below the LOWER row, the same bracket the flat strip gets.
     const strip: Rect = { left: 0, top: 0, width: 1200, height: 90 };
     const geom = helper.computePath({ parent: tab(0, 4), child: tab(200, 48), strip })!;
 
     expect(geom.sameRow).toBe(false);
-    // Parent bottom (34) → child top (48): the arc travels between rows.
-    expect(geom.d.startsWith('M 60 34')).toBe(true);
-    expect(geom.endY).toBe(48);
+    expect(geom.d.startsWith('M 60 34')).toBe(true); // parent BOTTOM
+    expect(geom.endY).toBe(78); // child BOTTOM, not its top
+    // Every control point clears the lower row by at least the minimum dip.
+    for (const y of controlYs(geom.d)) expect(y).toBeGreaterThanOrEqual(78 + helper.DIP_MIN_PX);
   });
 
-  it('draws upward when the child sits on the row ABOVE its parent', () => {
+  it('draws the same bracket when the child sits on the row ABOVE its parent', () => {
     const helper = loadLineageHelper();
     const strip: Rect = { left: 0, top: 0, width: 1200, height: 90 };
     const geom = helper.computePath({ parent: tab(0, 48), child: tab(200, 4), strip })!;
 
     expect(geom.sameRow).toBe(false);
-    expect(geom.d.startsWith('M 60 48')).toBe(true); // parent TOP edge
-    expect(geom.endY).toBe(34); // child bottom edge
+    expect(geom.d.startsWith('M 60 78')).toBe(true); // parent BOTTOM
+    expect(geom.endY).toBe(34); // child BOTTOM
+    // The parent's row is the lower one here, so that is what the curve clears.
+    for (const y of controlYs(geom.d)) expect(y).toBeGreaterThanOrEqual(78 + helper.DIP_MIN_PX);
   });
 
   it('skips an edge whose tab is scrolled out of the strip', () => {