fix(map): clamp layer fade progress to the valid range

A frame timestamp may precede performance.now sampled when hiding starts, producing negative progress and opacity above 1. Bound progress at both ends; cover the exact label failure and real MapLibre paint validation.

tests/browser/layer-visibility-fade.e2e.ts

@@ -48,6 +48,27 @@ async function openLayerFixture(page: Page, vectorPipes = false) {
   await page.getByRole("button", { name: /图层：管理地图图层/ }).click();
 }
 
+test("analysis label fade accepts a frame timestamp earlier than the visibility change", async ({ page }) => {
+  await openLayerFixture(page);
+  await page.evaluate(async () => {
+    const modulePath = "/src/features/workbench/map/layer-visibility.ts";
+    const { setMapLayersVisibility } = await import(modulePath);
+    const map = window.__waterNetworkMap!;
+    const errors: string[] = [];
+    map.on("error", (event) => {
+      if (event.error.message.includes("simulation-backend-analysis-label")) errors.push(event.error.message);
+    });
+    Object.assign(window, { analysisFadeErrors: errors });
+    const nativeFrame = window.requestAnimationFrame.bind(window);
+    window.requestAnimationFrame = (callback) => nativeFrame((time) => callback(time - 100));
+    try { setMapLayersVisibility(map, ["simulation-backend-analysis-label"], false); }
+    finally { window.requestAnimationFrame = nativeFrame; }
+  });
+  await expect.poll(() => page.evaluate(() => window.__waterNetworkMap!
+    .getLayoutProperty("simulation-backend-analysis-label", "visibility"))).toBe("none");
+  expect(await page.evaluate(() => (window as unknown as { analysisFadeErrors: string[] }).analysisFadeErrors)).toEqual([]);
+});
+
 for (const [label, layerId, property] of [
   ["管线", "pipes-flow", "line-opacity"],
   ["水库", "reservoirs-symbol", "icon-opacity"],