test: cover ts_rank window semantics

[Angelo9up]

Summary

• add a focused test for ts_rank trailing-window semantics
• verify the last element's normalized rank and front padding/output mask behavior

Validation

• pytest tests\test_tensor_factors.py -q — 9 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 a useful focused test: it pins the trailing-window s_rank behavior and the front-padding mask semantics without changing production code. I verified the targeted tensor-factor tests, full local pytest, ruff, diff check, and the GitHub CI matrix.