Fix period count in timeseries length mismatch error

[FLABDUL]

Summary

• interpolate the actual period count in PolarsTimeseries.from_vectors() length-mismatch errors
• assert the corrected message in the existing regression test
• make pull-request CI deterministic by using the locked Ruff version and reserving the protected Pages environment for release deployments

Closes #8

Validation

• uv run pytest -q -o log_cli=false — 260 passed, 3 skipped
• uv run ruff check .
• uv run ruff format --check .
• uv run pyrefly check
• just doctest — all 30 doctests passed locally; Windows then reports an existing case-insensitive Cash.rst/cash.rst autosummary collision

Shared CI repair

This PR contains the same CI prerequisite commit as #12 and #13: Ruff is read from uv.lock using the Node 24 action, and GitHub Pages deployment is split from the pull-request docs build. The shared commit only needs to land once; it can be dropped from later PRs after the first is merged.