fix(charts): preserve comparison types and draggable window headings

Keep current lines and historical scatter points in a dedicated comparison chart type. Expose floating headings and use idempotent placement so StrictMode no longer masks production overlap.

tests/browser/scada-comparison-chart.e2e.ts

@@ -9,8 +9,8 @@ const times = Array.from({ length: 13 }, (_, index) => `${String(9 + Math.floor(
 const block = {
   version: "answer-ui@1", block_id: "scada-week", phase: "final", fallback_text: "合成 SCADA 七日对照",
   spec: { root: "chart", elements: { chart: { type: "Chart", children: [], props: {
-    title: "压力与前七天同时刻对照（m）", chart_type: "line", y_axis_name: "压力（m）", x_axis_name: "时间（UTC0）",
-    x_data: times.map((time) => new Date(`2026-06-14T${time}:00+08:00`).toISOString().slice(11, 16)),
+    title: "压力与前七天同时刻对照（m）", chart_type: "time_comparison", y_axis_name: "压力（m）", x_axis_name: "时间",
+    x_data: times,
     series: [{ name: "当日三小时", type: "line", data: [34, 34.2, 33.8, 34, 33.5, 33, 32.5, 32, 31, 30, 29, 27, 25],
       sample_times: times.map((time) => new Date(`2026-06-14T${time}:00+08:00`).toISOString().replace(".000Z", "Z")) },
     ...Array.from({ length: 7 }, (_, index) => ({ name: `6月${13 - index}日`, type: "scatter",
@@ -45,7 +45,7 @@ for (const width of [1440, 375, 320]) {
     await panel.getByRole("button", { name: "发送 Agent 指令" }).click();
     const ui = panel.locator(".answer-ui-chart");
     await expect(ui).toBeVisible();
-    await expect(ui).toContainText("时间（UTC+8）");
+    await expect(ui.locator("svg text").filter({ hasText: /^时间$/ })).toBeVisible();
     await expect(ui.getByRole("tab", { name: "复合", exact: true })).toBeVisible();
     await expect(ui.getByLabel("显示的数据序列").getByRole("button")).toHaveCount(8);
     // Each historical day has exactly three unconnected SVG scatter symbols.