fix(ci): tighten slow bucket guardrails

tests/unit/scripts/run-test-bucket.test.js

@@ -0,0 +1,24 @@
+const { describe, expect, test } = require('bun:test');
+const path = require('node:path');
+const bucket = require('../../../scripts/run-test-bucket.js');
+
+describe('run-test-bucket', () => {
+  test('all declared slow tests still exist on disk', () => {
+    for (const relativePath of bucket.slowTests) {
+      const absolutePath = path.resolve(__dirname, '../../../', relativePath);
+      expect(Bun.file(absolutePath).exists()).resolves.toBe(true);
+    }
+  });
+
+  test('forces npm tests into the slow bucket', () => {
+    expect(bucket.shouldForceSlow('tests/npm/cli.test.js')).toBe(true);
+  });
+
+  test('keeps dist-independent javascript tests in the fast bucket', () => {
+    expect(bucket.shouldForceSlow('tests/unit/flag-parsing-simple.test.js')).toBe(false);
+  });
+
+  test('still forces dist-dependent tests into the slow bucket', () => {
+    expect(bucket.shouldForceSlow('tests/unit/config-dir-override.test.js')).toBe(true);
+  });
+});