Update runtime checks and installer behavior

scripts/check-node-version.mjs

@@ -0,0 +1,35 @@
+const MIN_NODE_VERSION = "20.18.1";
+
+function parseNodeVersion(version) {
+	const [major = "0", minor = "0", patch = "0"] = version.replace(/^v/, "").split(".");
+	return {
+		major: Number.parseInt(major, 10) || 0,
+		minor: Number.parseInt(minor, 10) || 0,
+		patch: Number.parseInt(patch, 10) || 0,
+	};
+}
+
+function compareNodeVersions(left, right) {
+	if (left.major !== right.major) return left.major - right.major;
+	if (left.minor !== right.minor) return left.minor - right.minor;
+	return left.patch - right.patch;
+}
+
+function isSupportedNodeVersion(version = process.versions.node) {
+	return compareNodeVersions(parseNodeVersion(version), parseNodeVersion(MIN_NODE_VERSION)) >= 0;
+}
+
+function getUnsupportedNodeVersionLines(version = process.versions.node) {
+	return [
+		`feynman requires Node.js ${MIN_NODE_VERSION} or later (detected ${version}).`,
+		"Switch to Node 20 with `nvm install 20 && nvm use 20`, or use the standalone installer:",
+		"curl -fsSL https://feynman.is/install | bash",
+	];
+}
+
+if (!isSupportedNodeVersion()) {
+	for (const line of getUnsupportedNodeVersionLines()) {
+		console.error(line);
+	}
+	process.exit(1);
+}