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. Rounds never overlap: a
round gives up the last round's checks if they are still waiting, which
happens only when a round starts early, after an interval change or
when the recovery probe finds a target answering. The probe still
checks every half second, giving up its previous checks, so they do not
pile up.

The frontend has its first unit tests, run by script/frontend-test with
Node's built-in test runner on a mocked clock. 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 +1,66 @@
+// 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 { test } from "node:test";
+import assert from "node:assert/strict";
+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 };
+
+// Mocks the clock for test t, so that a check lasting seconds takes no real
+// time, and replaces fetch with a target that answers after answerAfter
+// milliseconds of that clock, or never when answerAfter is Infinity. Both
+// are restored when the test ends.
+function mockTarget(t, answerAfter) {
+    t.mock.timers.enable({ apis: ["setTimeout", "Date"] });
+    t.mock.method(performance, "now", () => Date.now());
+    t.mock.method(
+        globalThis,
+        "fetch",
+        (url, { signal }) =>
+            new Promise((resolve, reject) => {
+                if (answerAfter !== Infinity) setTimeout(resolve, answerAfter);
+                signal.addEventListener("abort", () => reject(signal.reason));
+            }),
+    );
+}
+
+// The result of check if it has ended, otherwise "still waiting".
+function settled(check) {
+    return Promise.race([
+        check,
+        new Promise((resolve) => setImmediate(resolve, "still waiting")),
+    ]);
+}
+
+for (const interval of [10000, 30000]) {
+    const timeout = interval * 0.8;
+    // Over 3 seconds, which the timeout used to be capped at.
+    const slowAnswer = timeout - 1000;
+
+    test(`at a ${interval}ms interval, an answer after ${slowAnswer}ms is recorded with its real time`, async (t) => {
+        CONFIG.updateInterval = interval;
+        mockTarget(t, slowAnswer);
+        const check = measureLatency("https://target.test");
+        t.mock.timers.tick(slowAnswer);
+        assert.deepEqual(await settled(check), {
+            latency: slowAnswer,
+            error: null,
+        });
+    });
+
+    test(`at a ${interval}ms interval, a target that never answers is recorded as a timeout after ${timeout}ms`, async (t) => {
+        CONFIG.updateInterval = interval;
+        mockTarget(t, Infinity);
+        const check = measureLatency("https://target.test");
+        t.mock.timers.tick(timeout - 1);
+        assert.equal(await settled(check), "still waiting");
+        t.mock.timers.tick(1);
+        assert.deepEqual(await settled(check), {
+            latency: null,
+            error: "timeout",
+        });
+    });
+}