refactor: replace ArrayListUnmanaged with ArrayList per Zig 0.15 standards

- Renamed std.ArrayListUnmanaged to std.ArrayList across all source files
- Updated CLAUDE.md with Zig version standards rule
- Removed debug print from mixin test
- No API changes (allocator still passed to methods)

build.zig.zon

@@ -1,6 +1,6 @@
 .{
     .name = .pugz,
-    .version = "0.3.7",
+    .version = "0.3.8",
     .fingerprint = 0x822db0790e17621d, // Changing this has security and trust implications.
     .minimum_zig_version = "0.15.2",
     .dependencies = .{},