Reflect.hasField misses fields in fixed anonymous objects #222

Reflect.hasField returns false for fields in @:fixed anonymous objects, even though Reflect.field and Reflect.fields find those same fields.

Regression coverage first: test/regression/AnonFixedReflection, using the existing Main.hx/build.hxml/stdout.txt layout:

var value = @:fixed {answer: 42};
Sys.println(Reflect.field(value, "answer"));
Sys.println(Reflect.hasField(value, "answer"));
Sys.println(Reflect.fields(value).join(","));

Confirmed on Linux x86_64 with Haxe 4.3.3:

field: 42
hasField: false
fields: answer

Haxe Eval reports hasField: true for the same program. A missing field should still report false, and an existing field with a null value must report true.

include/hx/Anon.h provides AnonStruct1_obj through AnonStruct5_obj with __Field, __SetField and __GetFields overrides but no __HasField override. They inherit Object::__HasField from src/hx/Object.cpp, which always returns false. Ordinary Anon_obj implements its own existence query and does not have this problem.

Implement field existence consistently for every fixed anonymous-object arity, including null-valued fields and runtime-created field-name strings. This is relevant to structural typedef values emitted through the fixed-anon representation and to reflection-based validation or serialization that checks existence before reading.

The local regression is added before any implementation change. No runtime implementation changes are included.