Fix Windows MSVC iostream linking failure in BeamLatticeVoxelAcceleration tests

[Copilot]

This PR fixes a Windows MSVC build failure where the linker could not resolve std::basic_ostream symbols when building BeamLatticeVoxelAcceleration_tests.cpp. The error manifested as:

BeamLatticeVoxelAcceleration_tests.cpp.obj : error LNK2019: unresolved external symbol "class std::basic_ostream

Root Cause

The issue was caused by conflicting linker subsystem flags in the Windows build. The build logs showed both /SUBSYSTEM:WINDOWS and /subsystem:console being applied simultaneously:

/SUBSYSTEM:WINDOWS /ENTRY:mainCRTStartup /INCREMENTAL:NO /subsystem:console

This invalid configuration prevented proper iostream symbol linkage. The main CMakeLists.txt sets CMAKE_EXE_LINKER_FLAGS to include /SUBSYSTEM:WINDOWS globally, but test executables require console I/O for std::cout operations used extensively in the test suite.

Solution

Fixed subsystem conflict: Override the global Windows subsystem setting specifically for test executables:

set_target_properties(${BINARY} PROPERTIES LINK_FLAGS "/SUBSYSTEM:CONSOLE")

Explicit C++20 standard: Ensure unit tests match integration test configuration:

set_property(TARGET ${BINARY} PROPERTY CXX_STANDARD 20)
set_property(TARGET ${BINARY} PROPERTY CXX_STANDARD_REQUIRED ON)

Complete dependency chain: Added all required source files for the BeamLatticeVoxelAcceleration functionality:

• BeamLatticeVoxelAcceleration.h/cpp - main voxel acceleration implementation
• BeamLatticeVoxelAccelerationReference.h/cpp - reference implementation for testing
• BeamLatticeResource.h/cpp and BeamBVH.h/cpp - required dependencies

The issue was specific to this test file because it combines complex template usage (measureExecutionTime<Func>), extensive iostream formatting operations, C++17 structured bindings, and heavy std::cout usage that triggered more template instantiations than simpler test files.

Fixes #8.

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