For example, when createInstance() fails, impl_pollAPIMessages() dereferences instance , which is still a nullptr.

Enforce a no-warnings policy by compiling LLRI with -Werror, both locally and in GH actions

Simplify applications that don't require elaborate adapter selection by allowing device creation without an adapter, in such a way that createDevice() will simply select the default adapter (enumerateAdapters()[0]).

There's no consistent style in which comments are written in the repository. Establish a consistent rule for comments (we could for example require standard English rules - start with a capital letter, end with a dot), and enforce the ruling throughout the codebase.

validation_callback_desc should be available to all functions at all time. Currently the desc struct has to be stored in every object just to call it, which is tedious, decreases readability, and potentially increases memory usage.

Add the necessary DLLs and headers to use the much more up-to-date DX12 Agility SDK, which should help when debugging applications. https://devblogs.microsoft.com/directx/gettingstarted-dx12agility/

Output on DX builds:
T+ 1.313857 [ info ] [Initialization] : Found adapter Intel(R) Iris(R) Plus Graphics
T+ 1.316003 [ info ] [Initialization] : Vendor ID: 32902
T+ 1.317317 [ info ] [Initialization] : Adapter ID: 35418
T+ 1.317822 [ info ] [Initialization] : Adapter Type: Discrete

Output on VK builds (correct):
T+ 1.529683 [ info ] [Initialization] : Found adapter Intel(R) Iris(R) Plus Graphics
T+ 1.532333 [ info ] [Initialization] : Vendor ID: 32902
T+ 1.533437 [ info ] [Initialization] : Adapter ID: 35418
T+ 1.533789 [ info ] [Initialization] : Adapter Type: Integrated

in createInstance(), llri-vk uses raw strings in the unordered_set, switching to compile time hashed strings would result in a minor performance improvement

Volk reduces Vulkan's default loader overhead and can introduce as much as a 7% performance improvement: https://github.com/zeux/volk.

Enable (clang) address sanitiser in llri applications (especially unit tests) and require correct behaviour for completion

Introduce convenient aliases for linking against LLRI targets - llri::dx, llri::vk, and in the future llri::toolkit

Is your feature request related to a problem? Please describe.
Copyright is described on a per-file basis, with a starting and current year - A GitHub action should keep the current year updated by running on a yearly basis.

Describe the solution you'd like
Integrate https://github.com/FantasticFiasco/action-update-license-year into the repo's actions, configured in such a way that it checks every source file for the copyright notice.

Using https://lgtm.com/projects/g/Rythe-Interactive/Legion-LLRI/logs/languages/lang:cpp you can setup code quality reports which might be neat.

also using https://app.codecov.io/gh/Rythe-Interactive/Legion-LLRI you can setup code coverage.

am planning to set it up for rythe as well once i'm done reworking the build process

Introduce functions for each object type that enables the user to get the native pointers of underlying types. This allows users to integrate with libraries that expect native pointers. Users can then use llri::getImplementation() to cast the resulted pointers to the correct types.