test: warm cold dynamic imports before the timed cases

Under a full coverage run on every core, a test file's first cold import
(0.6-0.8 s alone) outran the 5 s case timeout and the half-done import
failed every later case in the file: the vitest lane went red in 2 of 5
runs with no defect. A beforeAll warm-up in the 4 files of that pattern;
5 of 5 full runs green after.

frontend/src/lib/diagnostics.session.dom.test.ts

@@ -4,7 +4,7 @@
 // consent), never lowers a raised one, tags every event it keeps, and its last seconds reach the
 // server before Stop. Its own file: `lib/diagnostics` and `lib/log` keep module-level state, so every
 // test imports fresh copies of both, as diagnostics.dom.test does.
-import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { afterEach, beforeAll, beforeEach, describe, expect, it, vi } from "vitest";
 
 const sent: { event: string; debug_session_id?: string }[][] = [];
 
@@ -45,6 +45,14 @@ const recording = (level: "DEBUG" | "TRACE", minutes = 60, over = {}) => ({
   ...over,
 });
 
+// Warm the transform cache once (see verifyEntry.dom.test.tsx): the first cold import costs ~0.8 s
+// alone and outran the 5 s case timeout under a full coverage run. Transformed code outlives
+// vi.resetModules, so each case still gets fresh module instances, only faster.
+beforeAll(async () => {
+  await import("./diagnostics");
+  await import("./log");
+}, 60_000);
+
 let current: typeof import("./diagnostics") | null = null;
 beforeEach(() => {
   vi.useFakeTimers();