Add blocking and vectorization boundary tests for LU and Cholesky

Pavel Guzenfeldrequested to merge
PavelGuzenfeld/eigen:test/lu-cholesky-boundary-coverage into master

Summary

  • Add boundary tests for PartialPivLU, FullPivLU, LLT, and LDLT at sizes near blocking boundaries (8, 16, 32, 64) and vectorization boundaries (PacketSize±1, 2*PacketSize±1, etc.)
  • Test reconstruction, solve, and invertibility at each boundary size
  • Test non-square matrices for FullPivLU at boundary sizes
  • Test RowMajor storage order for both LU and Cholesky at boundary sizes
  • Cover float, double, complex<float>, and complex<double> scalar types

Follows the boundary test coverage pattern from !2291 (merged) and !2290 (merged).

Test plan

  • All 9 lu_* tests pass (GCC 13, C++17)
  • All 9 cholesky_* tests pass (GCC 13, C++17)

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