test: cover __dirname, __filename, & require.cache in CJS TS file

## Problem

Two closed bugs left the CJS-scope-globals contract untested:

- [#694](#694) — Node 23.6+
began classifying `.ts` entrypoints as ESM in `type: "commonjs"`
packages, breaking `__dirname` and `__filename` with `ReferenceError:
__dirname is not defined in ES module scope`. Implicit fix landed in the
4.20/4.21 era; no paired regression test.
- [#726](#726) —
`require.cache` returned `undefined` in v4.20 after the loader refactor;
the maintainer reverted in v4.20.3 and explicitly noted it should land
as a test for the next release.

A future refactor of the CJS-classification path could silently
reintroduce either bug.

## Changes

One new `commonjs-mode-contracts.ts` test, iterated through both
`omitted type` and `explicit commonjs` package shapes (two process
spawns total), asserts that a `.ts` entrypoint sees:

- `__dirname` → fixture path
- `__filename` → fixture entry path
- `require.cache` → populated object after a sibling
`require('./dep.cjs')`

Author: privatenumber

tests/specs/commonjs-mode-contracts.ts

@@ -2,6 +2,7 @@ import {
 	describe, test, onTestFail, expect,
 } from 'manten';
 import { createFixture } from 'fs-fixture';
+import { outdent } from 'outdent';
 import type { NodeApis } from '../utils/tsx.js';
 import {
 	createPackageJson,
@@ -148,6 +149,51 @@ export const commonJsModeContracts = (node: NodeApis) => describe('CommonJS mode
 		}
 	});
 
+	// Regression guard for two closed bugs that lacked paired tests:
+	// - https://github.com/privatenumber/tsx/issues/694 -- Node 23.6+ classified
+	//   .ts entrypoints as ESM in CJS-shaped packages, breaking __dirname and
+	//   __filename ("not defined in ES module scope").
+	// - https://github.com/privatenumber/tsx/issues/726 -- v4.20 returned undefined
+	//   for require.cache; reverted in v4.20.3 without a paired test.
+	test('CJS-classified .ts entrypoint exposes __dirname, __filename, and require.cache', async () => {
+		for (const { label, packageJson } of commonJsModes) {
+			await using fixture = await createFixture({
+				'package.json': createPackageJson(packageJson),
+				'index.ts': outdent`
+					require('./dep.cjs');
+					console.log(JSON.stringify({
+						dirname: __dirname,
+						filename: __filename,
+						requireCacheType: typeof require.cache,
+						requireCacheHasKeys: Object.keys(require.cache).length > 0,
+					}));
+				`,
+				'dep.cjs': 'module.exports = { loaded: true };',
+			});
+
+			const result = await node.tsx(['index.ts'], fixture.path);
+			onTestFail(() => {
+				console.log(label, result);
+			});
+
+			expect({
+				failed: result.failed,
+				exitCode: result.exitCode,
+				stderr: result.stderr,
+			}).toEqual({
+				failed: false,
+				exitCode: 0,
+				stderr: '',
+			});
+			expect(JSON.parse(result.stdout)).toEqual({
+				dirname: fixture.path,
+				filename: fixture.getPath('index.ts'),
+				requireCacheType: 'object',
+				requireCacheHasKeys: true,
+			});
+		}
+	});
+
 	// tsx is intentionally more lenient than Node for ambiguous-`type` and
 	// explicit `"commonjs"` packages: a `.js`/`.ts` file that mixes ESM
 	// `import`/`export` syntax with explicit `require()` calls still runs,