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)

benchmarks/compiled/simple-2.zig

@@ -4,7 +4,7 @@ const helpers = @import("helpers.zig");
 pub const Data = struct {};
 
 pub fn render(allocator: std.mem.Allocator, _: Data) ![]const u8 {
-    var buf: std.ArrayListUnmanaged(u8) = .{};
+    var buf: std.ArrayList(u8) = .{};
     defer buf.deinit(allocator);
 
     try buf.appendSlice(allocator, "<h1>Header</h1><h2>Header2</h2><h3>Header3</h3><h4>Header4</h4><h5>Header5</h5><h6>Header6</h6><ul><li>item1</li><li>item2</li><li>item3</li></ul>");