Add Node smoke tests for @xterm/headless package

Verify CJS and ESM bundles load, accept writes, expose buffer API,
resize, and dispose. Add npm run test-headless-node script.

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

headless/test-node-load.mjs

@@ -0,0 +1,29 @@
+/**
+ * Smoke test: @xterm/headless loads and runs in Node.js (ESM + CJS).
+ */
+import { createRequire } from 'node:module';
+import { strictEqual } from 'node:assert';
+
+const require = createRequire(import.meta.url);
+
+function writeSync(term, data) {
+  return new Promise(resolve => term.write(data, resolve));
+}
+
+function lineEquals(term, index, text) {
+  strictEqual(term.buffer.active.getLine(index).translateToString(true), text);
+}
+
+const { Terminal: TerminalEsm } = await import('./lib-headless/xterm-headless.mjs');
+const termEsm = new TerminalEsm({ cols: 80, rows: 24 });
+await writeSync(termEsm, 'esm ok');
+lineEquals(termEsm, 0, 'esm ok');
+termEsm.dispose();
+
+const { Terminal: TerminalCjs } = require('./lib-headless/xterm-headless.js');
+const termCjs = new TerminalCjs({ cols: 40, rows: 10 });
+await writeSync(termCjs, 'cjs ok');
+lineEquals(termCjs, 0, 'cjs ok');
+termCjs.dispose();
+
+console.log('headless package: ESM and CJS load, write, and buffer read OK');