Add confidence fusion gate utilities and smoothing tests

[farmountain]

Summary

• add a lightweight fusion package that exposes sigmoid, clipping, and a stateful ConfidenceFusion gate
• provide lazy package exports so the fusion helpers are usable without importing heavy dependencies
• cover the mapping and smoothing behaviour with monotonicity, Lipschitz, and convergence unit tests

Testing

• pytest tests/test_alpha_monotone.py tests/test_alpha_bounds.py tests/test_alpha_smoothing.py

[farmountain]

Duplicate.