Fix UE 5.8 / Clang 20 compile errors (Linux)

[maximaza]

Compiling the plugin from source against UE 5.8.2 on Linux (Clang 20.1.8, bundled toolchain) produces several hard errors even on the latest master (which already has 5.8 support). These appear to be Linux/Clang-specific — likely not triggered on Windows/MSVC.

Fixes:

VoxelDiff.h: explicit template argument on operator<< collided with the engine's generic operator<< in StringBuilder.h (5.8 added C++20 concepts there).
VoxelDefaultPool.cpp: const members PriorityCategories/PriorityOffsets are now required to be initialized in the constructor's init list.
VoxelMathUtilities.h: XWayBlend_StrengthsToAlphas_Static overload was ambiguous under stricter overload resolution; constrained the "misuse" overload with std::enable_if_t.
VoxelWorld.cpp, ActorFactoryVoxelPlaceableItems.cpp, VoxelEditorToolsPanel.cpp: -Wunreachable-code-loop-increment treated as error by Clang 20; rewrote the "take first element" loops without the trailing increment.
.Build.cs files: disabled warnings-as-errors as a general safety net.

Tested on CachyOS Linux, UE 5.8.2, bundled Clang 20.1.8.