- Change from 3-way split (~1900 lines) to 6-way split (~750-825 lines)
- Extract all helper functions into interactions_helpers.hpp
- Remove monolithic interactions.cpp in favor of split parts
- Each part contains ~27-28 tests
- All 164 tests pass on Windows with smaller compilation units

- interactions_helpers.hpp now contains only forward declarations (92 lines)
- interactions_helpers.cpp contains implementations (1825 lines)
- Helpers compiled as static library, test parts link against it
- This prevents MSVC from processing all helpers for each test file

Even the single helpers.cpp was causing heap exhaustion.
Split into 5 parts to keep each file small enough for MSVC.

The 64-bit host compiler has more addressable memory than the 32-bit
host, which helps avoid C1060 (out of heap space) errors when compiling
template-heavy code like JSON object construction.

SSE tests can be flaky on CI when running in parallel due to
port binding conflicts. Running them serially improves reliability.