[Feature] DaedalusSymbol: Expose SetInstance/GetInstance to support MEM_AssignInst

[JaXt0r]

Ikarus's MEM_AssignInst assigns a transient instance to a named symbol so that subsequent script code accessing that symbol operates on the custom instance. This requires writing a DaedalusInstance back into a symbol's instance slot, which is not currently possible from C#.

Reference OpenGothic (directmemory.cpp):

void DirectMemory::mem_assigninst(int index, int ptr) {
    auto* sym = vm.find_symbol_by_index(uint32_t(index));
    sym->set_instance(std::make_shared<memory_instance>(*this, ptr32_t(ptr)));
}

Requested C# API:

• DaedalusSymbol.SetInstance()
• DaedalusSymbol.GetInstance()