Edge cases: singular / ill-conditioned matrices

[tec-eli]

Description

Edge-case tests for singular, nearly-singular, and ill-conditioned matrices across the
dense decomposition algorithms.

• Scope: tests/edge_cases/singular.rs, tests/edge_cases/condition.rs

Tasks

• LU on rank-deficient matrix returns Err, not NaN
• Cholesky on non-positive-definite matrix returns Err
• SVD on rank-deficient matrix produces zero singular values at the end
• Feed a matrix with κ >> 1e7 into LU/QR/SVD; document expected accuracy loss
• Verify the library returns a result (not a panic) at extreme condition numbers