build: dual CJS/ESM output, ES2020 target, Node 18 floor

"exports" gains "import" and "types" next to "require": import "alex2node" failed with
ERR_PACKAGE_PATH_NOT_EXPORTED. scripts/build.mjs compiles src/ to dist/cjs, dist/esm and dist/types in a staging
directory and swaps them in only when both compiler runs passed. "main" and "types" follow for resolvers that do
not read "exports". Target ES6 -> ES2020, "engines": node >= 18, relative imports carry ".js" for Node's ESM loader.
uuid gives way to crypto.randomUUID(); mqtt is the only runtime dependency left.

Tests load the package by name. New: both entry points, the modules the builds import, dist/ equal to a fresh
build, a round trip on the ES module build, and two compiled type fixtures in place of the .d.ts regexes.
22 tests pass in 5.7 s. dist/: 16 files, 79,711 B -> 25 files, 132,291 B; the npm tarball is 25,125 B.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

scripts/build.mjs

@@ -0,0 +1,49 @@
+// npm run build - the only writer of dist/.
+//
+//   node scripts/build.mjs [--out <dir>]      <dir> defaults to dist/ in the package
+//
+// Two compiler runs over src/: CommonJS with the declarations (cjs/, types/), then ES modules (esm/). Both write to a
+// staging directory that replaces the three directories only after both runs passed, so a failed build leaves the
+// tracked dist/ as it was. Files of sources that no longer exist go with the old directories.
+import { spawnSync } from "node:child_process";
+import { mkdirSync, mkdtempSync, renameSync, rmSync, writeFileSync } from "node:fs";
+import { createRequire } from "node:module";
+import { dirname, join, resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const root = resolve(dirname(fileURLToPath(import.meta.url)), "..");
+const tsc = createRequire(import.meta.url).resolve("typescript/bin/tsc");
+
+function outputDirectory(args) {
+  const at = args.indexOf("--out");
+  if (at === -1) return join(root, "dist");
+  if (!args[at + 1]) throw new Error("--out needs a directory");
+  return resolve(args[at + 1]);
+}
+
+function compile(project, options) {
+  const run = spawnSync(process.execPath, [tsc, "-p", join(root, project), ...options], { cwd: root, stdio: "inherit" });
+  if (run.error) throw run.error;
+  if (run.status !== 0) throw new Error(`tsc -p ${project} failed, ${out} is unchanged`);
+}
+
+let out;
+let staging;
+try {
+  out = outputDirectory(process.argv.slice(2));
+  mkdirSync(out, { recursive: true });
+  staging = mkdtempSync(join(out, ".build-"));
+  compile("tsconfig.cjs.json", ["--outDir", join(staging, "cjs"), "--declarationDir", join(staging, "types")]);
+  compile("tsconfig.esm.json", ["--outDir", join(staging, "esm")]);
+  // The package is "type": "commonjs"; this marks the .js files under esm/ as ES modules.
+  writeFileSync(join(staging, "esm", "package.json"), JSON.stringify({ type: "module" }, null, 2) + "\n");
+  for (const part of ["cjs", "esm", "types"]) {
+    rmSync(join(out, part), { recursive: true, force: true });
+    renameSync(join(staging, part), join(out, part));
+  }
+} catch (err) {
+  console.error(`build: ${err.message}`);
+  process.exitCode = 1;
+} finally {
+  if (staging) rmSync(staging, { recursive: true, force: true });
+}