test: cover ts_cov window semantics

[Angelo9up]

Summary

• add direct coverage for ts_cov trailing-window semantics
• verify sample covariance values and front-padding output masks

Validation

• pytest tests\test_tensor_factors.py -q — 10 passed
• pytest -q — all tests passed
• ruff check tests\test_tensor_factors.py — passed
• git diff --check — passed (Git only reported the Windows LF/CRLF conversion warning)

This is a focused test-only change; no production implementation is modified.

[initial-d]

Thanks, @Angelo9up. This is another focused and useful primitive-semantics test: it verifies s_cov uses trailing sample covariance and preserves the expected front-padding mask behavior. I checked the math against the implementation and verified targeted tests, full local pytest, ruff, diff check, and the GitHub CI matrix.