Fix pointer offset loss in OwnerPtr::Cast and ShadowPtr::Shadow under multiple inheritance

[Copilot]

When casting through a multiple-inheritance hierarchy, converting VT* → void* → VT2* via static_cast discards the pointer offset, producing an incorrect pointer and corrupting all member access on the result.

Root cause: ControlBlock::Data stored the pointer as void*, and Get() re-derived the typed pointer via static_cast<VT*>(void*). This loses the offset adjustment that compilers apply when navigating multiple-inheritance bases.

Fix: dual-pointer design (mirrors std::shared_ptr)

• m_pTyped — a correctly-typed VT* added directly to both OwnerPtr and ShadowPtr, used exclusively for object access (Get(), ->, *)
• ControlBlock::Data — unchanged; continues to hold the original allocation pointer for the deleter and acts as the liveness indicator for ShadowPtr

Key method updates:

• Get() returns m_pTyped directly — no more static_cast from void*
• Cast<VT2>() stores the dynamic_cast<VT2*>(m_pTyped) result in the new OwnerPtr's m_pTyped
• Shadow<VT2>() propagates static_cast<VT2*>(m_pTyped) into the new ShadowPtr
• All constructors, move/assignment operators, Transfer(), Release(), Swap(), Destroy(), and EnableShadowFromThis::ShadowFromThis() updated accordingly

Example (previously broken):

class BaseObject {
public:
    BaseObject(int v) : m_value(v) {}
    int GetValue() const { return m_value; }
private:
    int m_value;
};

class DerivedObject : public BaseObject, public SomeOtherBase { ... };

OwnerPtr<DerivedObject> d = OwnerPtr<DerivedObject>::Create(42);
OwnerPtr<BaseObject> b = d.Cast<BaseObject>();
b->GetValue(); // was: UB / wrong value; now: 42

Pre-existing compile errors also fixed (were blocking the build on GCC/Clang with C++17):

• Duplicate using DeleterType = DT in OwnerPtr
• Ordered comparisons of T* vs std::nullptr_t in ShadowPtr and OwnerPtr comparison operators (replaced with static_cast<ConstPointer>(nullptr))
• Matching test assertion expressions updated to use a typed null pointer variable