Consolidate engine interfaces and test registration

[mtcorread]

Summary

• Consolidate internal engine interfaces and remove obsolete helper APIs.
• Centralize event lookup schemas and simplify loader and event-logging paths.
• Consolidate public C++ test registration and retain public regression fixtures.

Verification

• CMake configure and single-worker build completed successfully with MPI enabled and profiling disabled.
• Public serial CTest: 61/61 passed.
• Public MPI CTest: 68/68 passed.
• Integrated CTest: 31/31 passed.
• Serial and two-rank MPI regression scenarios passed.
• Duplicate seed-identity rejection behaved as expected.
• Baseline-versus-integrated HDF5 comparison: 10/10 passed; dataset-level comparisons were exact.
• git diff --check passed.