Move small-sigma Gaussian blur return to a helper

[fyrsta7]

Summary

This moves the small-sigma identity return used by GaussianBlurParameters::new_from_sigma into a cold helper, leaving the normal positive-sigma path as the straight-line path.

Why this can improve performance

The sigma <= 0.0 case returns an identity kernel and is not the normal blur construction path. Keeping that path in a cold helper lets the common Gaussian-kernel setup stay a little smaller and more direct.

Validation

• git diff --check
• cargo check

Benchmark

Current machine, release microbenchmark, 9 samples, median:

Benchmark	Base	This change	Delta
1024 GaussianBlurParameters::new_from_sigma calls	5,228.287 ns	5,214.858 ns	0.26% faster

[fyrsta7]

Closing this draft because follow-up benchmarking did not show a large enough performance gain to justify this as an upstream performance PR.