Align test expectation with sample scaling and silence backend warnings

[SauersML]

Summary

• expect the Linfa consistency test to use the sample-standardized explained variance and drop an unnecessary mutable binding
• scope ndarray backend imports so they are only pulled in for non-faer builds and mark helper items to avoid dead-code warnings when faer is active

Testing

• cargo test --test pca_tests
• cargo check --all-features (fails: intel-mkl build script redefines MKL_CONFIG)

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https://chatgpt.com/codex/tasks/task_e_68d3821db36c832ebce0fae02e667590