perf(web): restore last known dashboard tiles

apps/web/src/lib/client/dashboard-refresh.test.ts

@@ -1,6 +1,7 @@
 import { describe, expect, test } from "vitest";
 import {
   attachDashboardRefreshLifecycle,
+  createDashboardTileSnapshotStore,
   createDashboardTileBackoff,
   createDashboardRequestAborter,
   runDashboardHydrationQueue,
@@ -131,6 +132,67 @@ describe("dashboard refresh lifecycle", () => {
     backoff.recordSuccess("telemetry:infra-ram");
     expect(backoff.canAttempt("telemetry:infra-ram", 107_000)).toBe(true);
   });
+
+  test("stores only ready tile snapshots and restores them for matching revisions", () => {
+    const storage = createMemoryStorage();
+    let now = 1_000;
+    const store = createDashboardTileSnapshotStore(storage, {
+      now: () => now,
+    });
+
+    store.saveReadyTile({
+      currentRevisionId: "revision-a",
+      response: {
+        state: "ready",
+        tile: { kind: "telemetry", id: "infra-ram" },
+        item: {
+          id: "infra-ram",
+          label: "Infra RAM",
+          value: { kind: "percent", value: 42 },
+          detail: "live",
+          severity: "ok",
+        },
+      },
+      schemaVersion: "dashboard.v1",
+    });
+    store.saveTile({
+      currentRevisionId: "revision-a",
+      response: {
+        state: "not_found",
+        tile: { kind: "telemetry", id: "missing" },
+        message: "Missing",
+      },
+      schemaVersion: "dashboard.v1",
+    });
+
+    now = 61_000;
+
+    const restored = store.restore({
+      currentRevisionId: "revision-a",
+      schemaVersion: "dashboard.v1",
+    });
+
+    expect(restored).toEqual([
+      {
+        ageMs: 60_000,
+        response: {
+          state: "ready",
+          tile: { kind: "telemetry", id: "infra-ram" },
+          item: {
+            id: "infra-ram",
+            label: "Infra RAM",
+            value: { kind: "percent", value: 42 },
+            detail: "live - stale 60s",
+            severity: "stale",
+          },
+        },
+      },
+    ]);
+    expect(store.restore({
+      currentRevisionId: "revision-b",
+      schemaVersion: "dashboard.v1",
+    })).toEqual([]);
+  });
 });
 
 async function waitFor(predicate: () => boolean) {
@@ -141,3 +203,27 @@ async function waitFor(predicate: () => boolean) {
 
   throw new Error("condition was not met");
 }
+
+function createMemoryStorage(): Storage {
+  const values = new Map<string, string>();
+  return {
+    get length() {
+      return values.size;
+    },
+    clear() {
+      values.clear();
+    },
+    getItem(key) {
+      return values.get(key) ?? null;
+    },
+    key(index) {
+      return [...values.keys()][index] ?? null;
+    },
+    removeItem(key) {
+      values.delete(key);
+    },
+    setItem(key, value) {
+      values.set(key, value);
+    },
+  };
+}