fix: decouple timeline from workspace layout

The regression recurred because a floating timeline height was still exported as shared layout space and its z-index exceeded artifacts and notices. Reserve zero layout height and define the timeline below artifact and notification overlays.

src/features/workbench/operational-timeline/operational-timeline-model.test.ts

@@ -4,6 +4,7 @@ import {
   canCenterTimelineBesideMapScale,
   clusterOperationalEvents,
   createOperationalEvents,
+  OPERATIONAL_TIMELINE_LAYOUT,
   resolveTimelineScaleLayout
 } from "./operational-timeline-model";
 
@@ -15,6 +16,10 @@ const workOrder: ScheduledConditionItem = {
 };
 
 describe("operational timeline model", () => {
+  it("does not reserve workspace layout space for the floating timeline", () => {
+    expect(OPERATIONAL_TIMELINE_LAYOUT.overlayReservedHeight).toBe(0);
+  });
+
   it("creates linked plan and actual events for active work orders", () => {
     const events = createOperationalEvents([workOrder]);
     expect(events.map((event) => event.lane)).toEqual(["plan", "actual"]);