Fix dense Cholesky refinement residual storage

DenseCholesky accepts a column-major lower triangle. Map that storage
explicitly and use its lower self-adjoint view for refinement residuals.

Poison unused triangles in the existing backend and size tests. Compute
the right-hand side before poisoning the input. Direct refiner tests
retain a full-matrix reference and factorization. Preserve tolerances
and iteration limits.

Issue: https://github.com/ceres-solver/ceres-solver/issues/863

Codex used.

Change-Id: I06d272ac382de5aecd6805a02d5f9c996aa88a4f