Update runtime checks and installer behavior

tests/node-version.test.ts

@@ -0,0 +1,35 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import {
+	MIN_NODE_VERSION,
+	ensureSupportedNodeVersion,
+	getUnsupportedNodeVersionLines,
+	isSupportedNodeVersion,
+} from "../src/system/node-version.js";
+
+test("isSupportedNodeVersion enforces the exact minimum floor", () => {
+	assert.equal(isSupportedNodeVersion("20.18.1"), true);
+	assert.equal(isSupportedNodeVersion("20.19.0"), true);
+	assert.equal(isSupportedNodeVersion("21.0.0"), true);
+	assert.equal(isSupportedNodeVersion("20.18.0"), false);
+	assert.equal(isSupportedNodeVersion("18.17.0"), false);
+});
+
+test("ensureSupportedNodeVersion throws a guided upgrade message", () => {
+	assert.throws(
+		() => ensureSupportedNodeVersion("18.17.0"),
+		(error: unknown) =>
+			error instanceof Error &&
+			error.message.includes(`Node.js ${MIN_NODE_VERSION}`) &&
+			error.message.includes("nvm install 20 && nvm use 20") &&
+			error.message.includes("https://feynman.is/install"),
+	);
+});
+
+test("unsupported version guidance reports the detected version", () => {
+	const lines = getUnsupportedNodeVersionLines("18.17.0");
+
+	assert.equal(lines[0], "feynman requires Node.js 20.18.1 or later (detected 18.17.0).");
+	assert.ok(lines.some((line) => line.includes("curl -fsSL https://feynman.is/install | bash")));
+});