feat(nexus): add TopStrip composite for layout overhaul (phase 8)

48px sticky top strip per docs/specs/2026-04-11-nexus-layout-overhaul.md
§4.2. Composes ModeBreadcrumb (left) + CmdKButton and GlobalMicButton
(right) inside a <header aria-label="Top bar"> landmark.

Completes the frame component set for Phase 8. The next task (task 6)
rewrites Layout.tsx to mount IconRail + TopStrip as the new global
chrome and delete the old sidebar/ChatPanel/PropertiesPanel/
BreadcrumbBar combination.

Part of Phase 8 of the Nexus layout overhaul (task 5 of 7).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/frame/TopStrip.test.tsx

@@ -0,0 +1,76 @@
+// @vitest-environment jsdom
+
+import { act } from "react";
+import { createRoot } from "react-dom/client";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { MemoryRouter } from "@/lib/router";
+import { TopStrip } from "./TopStrip";
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+(globalThis as any).IS_REACT_ACT_ENVIRONMENT = true;
+
+// Same stub as IconRail / ModeBreadcrumb tests — @/lib/router's Link
+// (and useLocation consumers) trigger CompanyContext resolution.
+vi.mock("@/context/CompanyContext", () => ({
+  useCompany: () => ({
+    companies: [],
+    selectedCompany: null,
+    selectedCompanyId: null,
+    setSelectedCompanyId: () => {},
+    selectionSource: null,
+    loading: false,
+  }),
+}));
+
+describe("TopStrip", () => {
+  let container: HTMLDivElement;
+  let root: ReturnType<typeof createRoot> | null = null;
+
+  beforeEach(() => {
+    container = document.createElement("div");
+    document.body.appendChild(container);
+    root = null;
+  });
+
+  afterEach(() => {
+    if (root) {
+      act(() => { root!.unmount(); });
+      root = null;
+    }
+    if (container.parentNode) container.remove();
+  });
+
+  function render(pathname: string) {
+    root = createRoot(container);
+    act(() => {
+      root!.render(
+        <MemoryRouter initialEntries={[pathname]}>
+          <TopStrip />
+        </MemoryRouter>
+      );
+    });
+  }
+
+  it("renders the ModeBreadcrumb with derived segments", () => {
+    render("/NEX/assistant");
+    const segment = container.querySelector("[data-testid='mode-breadcrumb-segment']");
+    expect(segment?.textContent?.trim()).toBe("ASSISTANT");
+  });
+
+  it("renders the CmdK button", () => {
+    render("/NEX/assistant");
+    expect(container.querySelector("button[aria-label='Open command palette']")).not.toBeNull();
+  });
+
+  it("renders the global mic button", () => {
+    render("/NEX/assistant");
+    expect(container.querySelector("button[aria-label='Voice']")).not.toBeNull();
+  });
+
+  it("is wrapped in a header element for landmark semantics", () => {
+    render("/NEX/assistant");
+    const header = container.querySelector("header");
+    expect(header).not.toBeNull();
+    expect(header?.getAttribute("aria-label")).toBe("Top bar");
+  });
+});

ui/src/components/frame/TopStrip.tsx

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+import { CmdKButton } from "./CmdKButton";
+import { GlobalMicButton } from "./GlobalMicButton";
+import { ModeBreadcrumb } from "./ModeBreadcrumb";
+
+/**
+ * TopStrip — the 48px top bar composed into the global frame in Layout.tsx
+ * (Task 6). Renders the mode breadcrumb on the left and the ⌘K + global
+ * mic buttons on the right. Charcoal bottom border, pure black background,
+ * sticky at the top of the main column.
+ *
+ * Per docs/specs/2026-04-11-nexus-layout-overhaul.md §4.2.
+ */
+export function TopStrip() {
+  return (
+    <header
+      aria-label="Top bar"
+      className="sticky top-0 z-30 flex h-12 shrink-0 items-center justify-between border-b border-border bg-background px-6"
+    >
+      <ModeBreadcrumb />
+      <div className="flex items-center gap-4">
+        <CmdKButton />
+        <GlobalMicButton state="idle" />
+      </div>
+    </header>
+  );
+}