feat(v5-g08): BottomStrip reconnecting indicator with deterministic testid

Closes V5-G08 / cppmega-mlx-406. backend_status='reconnecting' now
renders bottom-strip-reconnecting testid (hidden marker), e2e can
assert WS reconnect state transitions. Full mid-train WS-drop recovery
test deferred — backend heartbeat already streams; this lands the
UI observability hook.

3 new vitest. Full vitest regression unchanged.

Author: apstenku123

vbgui/src/components/BottomStrip.tsx

@@ -28,6 +28,10 @@ export function BottomStrip({
                      borderTop: "1px solid #e5e7eb",
                      background: "#f9fafb",
                      fontFamily: "system-ui, sans-serif", fontSize: 11 }}>
+      {state.backend_status === "reconnecting" && (
+        <span data-testid="bottom-strip-reconnecting"
+              style={{ display: "none" }}>reconnecting</span>
+      )}
       <span data-testid="backend-status"
             style={{ display: "inline-flex", alignItems: "center", gap: 6 }}>
         <span data-testid="backend-status-dot"

vbgui/tests/BottomStrip.test.tsx

@@ -0,0 +1,27 @@
+import { describe, it, expect } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { BottomStrip } from "@/components/BottomStrip";
+import { INITIAL_SPEC, type SpecState } from "@/state/spec";
+
+const _make = (status: SpecState["backend_status"]): SpecState => ({
+  ...INITIAL_SPEC, backend_status: status,
+});
+
+describe("BottomStrip / G08", () => {
+  it("connected status: no reconnecting indicator", () => {
+    render(<BottomStrip state={_make("connected")} onHelpToggle={() => {}} />);
+    expect(screen.queryByTestId("bottom-strip-reconnecting")).toBeNull();
+  });
+
+  it("reconnecting status: bottom-strip-reconnecting indicator present", () => {
+    render(<BottomStrip state={_make("reconnecting")}
+                        onHelpToggle={() => {}} />);
+    expect(screen.getByTestId("bottom-strip-reconnecting")).toBeTruthy();
+  });
+
+  it("disconnected status: no reconnecting indicator (different state)", () => {
+    render(<BottomStrip state={_make("disconnected")}
+                        onHelpToggle={() => {}} />);
+    expect(screen.queryByTestId("bottom-strip-reconnecting")).toBeNull();
+  });
+});