swarm-ui: test the rain arithmetic and the swarm name loader

The rain's column maths and fade colour move from MatrixRain into
shell/rain.ts, and the request-once swarm name loader from useSwarmName
into shell/swarm-name.ts. Both are plain functions with their input
passed in, so tests cover them; the components keep the DOM and canvas
work.

frontend/packages/swarm-ui/test/swarm-name.test.mjs

@@ -0,0 +1,60 @@
+// The swarm name loader: one request however many callers, and a name that is
+// absent, empty, or failed to load all answer null. Run by `npm test`.
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { createSwarmNameLoader } from "../src/shell/swarm-name.ts";
+
+// A fetch that answers `body` as an ok response, counting its calls.
+function fetching(body, ok = true) {
+  const f = async (url) => {
+    f.calls.push(url);
+    return { ok, json: async () => body };
+  };
+  f.calls = [];
+  return f;
+}
+
+test("the loader answers the configured name", async () => {
+  const f = fetching({ name: "constellation" });
+  assert.equal(await createSwarmNameLoader(f)(), "constellation");
+  assert.deepEqual(f.calls, ["/api/swarm"]);
+});
+
+test("every caller shares one request", async () => {
+  const f = fetching({ name: "constellation" });
+  const load = createSwarmNameLoader(f);
+  const names = await Promise.all([load(), load(), load()]);
+  assert.deepEqual(names, ["constellation", "constellation", "constellation"]);
+  assert.equal(f.calls.length, 1);
+});
+
+test("a null, empty or missing name answers null", async () => {
+  for (const body of [{ name: null }, { name: "" }, {}]) {
+    assert.equal(await createSwarmNameLoader(fetching(body))(), null);
+  }
+});
+
+test("an error response answers null", async () => {
+  assert.equal(
+    await createSwarmNameLoader(fetching({ name: "x" }, false))(),
+    null,
+  );
+});
+
+test("a failed request answers null rather than throwing", async () => {
+  const load = createSwarmNameLoader(async () => {
+    throw new Error("offline");
+  });
+  assert.equal(await load(), null);
+});
+
+test("a failed request is remembered, not retried by later callers", async () => {
+  let calls = 0;
+  const load = createSwarmNameLoader(async () => {
+    calls += 1;
+    throw new Error("offline");
+  });
+  await load();
+  await load();
+  assert.equal(calls, 1);
+});