Target check timeout is 80% of the refresh interval (closes #78)

Each target check now times out after 80% of the refresh interval, 24
seconds at 30 seconds, where it was capped at 3 seconds, so slow, far
targets are recorded with their real time. A round asked for while the
last one's checks are still waiting, after an interval change or by the
recovery probe, is skipped, so rounds never overlap; the recovery probe
starts no new checks while its last ones wait.

The frontend has its first unit tests, run by script/frontend-test with
Node's built-in test runner. index.html now links src/styles.css, which
src/main.js imported, since Node cannot import CSS.

Model: opus-5-5

test/unit/main.test.js

@@ -0,0 +1,52 @@
+// Unit tests for src/main.js, run by script/frontend-test with Node's
+// built-in test runner. Importing the module does not start the page.
+
+import { after, before, test } from "node:test";
+import assert from "node:assert/strict";
+import { createServer } from "node:http";
+import { CONFIG, measureLatency } from "../../src/main.js";
+
+// measureLatency writes timeouts to the debug log, which looks for its
+// panel in the page. There is no page here.
+globalThis.document = { getElementById: () => null };
+
+// A target that answers after the number of milliseconds in the path,
+// e.g. /600.
+let server;
+let target;
+
+before(async () => {
+    server = createServer((req, res) => {
+        const delay = Number(new URL(req.url, "http://x").pathname.slice(1));
+        setTimeout(() => res.end(), delay);
+    });
+    await new Promise((resolve) => server.listen(0, "127.0.0.1", resolve));
+    target = `http://127.0.0.1:${server.address().port}`;
+});
+
+after(() => {
+    server.closeAllConnections();
+    server.close();
+});
+
+test("the timeout is 80% of the refresh interval", () => {
+    CONFIG.updateInterval = 30000;
+    assert.equal(CONFIG.requestTimeout, 24000);
+    CONFIG.updateInterval = 3000;
+    assert.equal(CONFIG.requestTimeout, 2400);
+});
+
+test("an answer within the timeout is recorded with its real time, a later one as a timeout", async () => {
+    // 600ms is past the 400ms timeout of a 500ms interval...
+    CONFIG.updateInterval = 500;
+    assert.deepEqual(await measureLatency(`${target}/600`), {
+        latency: null,
+        error: "timeout",
+    });
+
+    // ...and within the 1200ms timeout of a 1500ms interval.
+    CONFIG.updateInterval = 1500;
+    const { latency, error } = await measureLatency(`${target}/600`);
+    assert.equal(error, null);
+    assert.ok(latency >= 600 && latency < 1200, `latency ${latency}ms`);
+});