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)

src/view_engine.zig

@@ -311,7 +311,7 @@ pub const ViewEngine = struct {
 
         // Normalize the path (resolve ".." and ".")
         // We need to handle this manually since std.fs.path.resolve needs absolute paths
-        var components = std.ArrayListUnmanaged([]const u8){};
+        var components = std.ArrayList([]const u8){};
         defer components.deinit(allocator);
 
         var iter = std.mem.splitScalar(u8, joined, '/');