fix(ci): cross-platform test runner for Windows glob expansion

npm scripts pass `tests/*.test.cjs` to node/c8 as a literal string on
Windows (PowerShell/cmd don't expand globs). Adding `shell: bash` to CI
steps doesn't help because c8 spawns node as a child process using the
system shell. Use a Node script to enumerate test files cross-platform.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/run-tests.cjs

@@ -0,0 +1,25 @@
+#!/usr/bin/env node
+// Cross-platform test runner — resolves test file globs via Node
+// instead of relying on shell expansion (which fails on Windows PowerShell/cmd).
+'use strict';
+
+const { readdirSync } = require('fs');
+const { join } = require('path');
+const { execFileSync } = require('child_process');
+
+const testDir = join(__dirname, '..', 'tests');
+const files = readdirSync(testDir)
+  .filter(f => f.endsWith('.test.cjs'))
+  .sort()
+  .map(f => join('tests', f));
+
+if (files.length === 0) {
+  console.error('No test files found in tests/');
+  process.exit(1);
+}
+
+try {
+  execFileSync(process.execPath, ['--test', ...files], { stdio: 'inherit' });
+} catch (err) {
+  process.exit(err.status || 1);
+}