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

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

src/archiveExtensions.js

@@ -0,0 +1,14 @@
+import path from 'node:path';
+
+const DOUBLE_EXTENSIONS = ['tar.gz', 'tar.bz2', 'tar.7z'];
+
+export function extractExtension(filename) {
+  const lower = filename.toLowerCase();
+  const match = DOUBLE_EXTENSIONS.find((ext) => lower.endsWith(`.${ext}`));
+  return match ?? path.extname(filename).slice(1).toLowerCase();
+}
+
+export function stripExtension(filename) {
+  const ext = extractExtension(filename);
+  return filename.slice(0, filename.length - ext.length - 1);
+}