Feature/pseudosqrt tests

[losterbr]

Summary

Improve pseudoSqrt and rankReducedSqrt test coverage and guard empty inputs.

Changes

• Add coverage for all pseudoSqrt salvage algorithms:
  • None
  • Spectral
  • Hypersphere
  • LowerDiagonal
  • Higham
  • Principal
• Add coverage for rank reduction, validation, singular matrices, zero matrices, and oversized rank limits.
• Verify PSD reconstruction, diagonal preservation, finite outputs, and Principal tolerance behavior.
• Reject empty matrices explicitly in both public APIs.
• Correct the normalization diagnostic from “pseudo rows” to “pseudo columns”.

[coveralls]

coverage: 78.421% (+0.2%) from 78.251% — losterbr:feature/pseudosqrt-tests into lballabio:master