Module heap_variants 

Single source of truth for HeapValue variants.

define_heap_types! generates:

• HeapValue enum
• HeapKind enum (discriminant)
• HeapValue::kind() method
• HeapValue::is_truthy() method
• HeapValue::type_name() method

equals(), structural_eq(), and Display remain hand-written because they have complex per-variant logic (e.g. cross-type numeric comparison).