feat(archive): add double-extension-aware filename parsing helper

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

test/archiveExtensions.test.js

@@ -0,0 +1,34 @@
+import { describe, it, expect } from 'vitest';
+import { extractExtension, stripExtension } from '../src/archiveExtensions.js';
+
+describe('extractExtension', () => {
+  it('returns the simple extension for a normal filename', () => {
+    expect(extractExtension('photo.PNG')).toBe('png');
+  });
+
+  it('recognizes tar.gz as a single compound extension', () => {
+    expect(extractExtension('backup.TAR.GZ')).toBe('tar.gz');
+  });
+
+  it('recognizes tar.bz2 as a single compound extension', () => {
+    expect(extractExtension('backup.tar.bz2')).toBe('tar.bz2');
+  });
+
+  it('recognizes tar.7z as a single compound extension', () => {
+    expect(extractExtension('backup.tar.7z')).toBe('tar.7z');
+  });
+
+  it('does not treat an unrelated double extension as compound', () => {
+    expect(extractExtension('archive.zip.bak')).toBe('bak');
+  });
+});
+
+describe('stripExtension', () => {
+  it('strips a simple extension', () => {
+    expect(stripExtension('photo.png')).toBe('photo');
+  });
+
+  it('strips a compound tar.gz extension, keeping the rest of the name intact', () => {
+    expect(stripExtension('my.backup.tar.gz')).toBe('my.backup');
+  });
+});