Add app source and tests

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

lib/Peach/about/internal/application/Pair.dart

@@ -0,0 +1,8 @@
+/// A generic two-slot carrier, used where a call needs to hand back two values
+/// without earning a named model (e.g. a resolved command plus its headers).
+class Pair<K, V> {
+  final K key;
+  final V value;
+
+  const Pair(this.key, this.value);
+}