refactor: разделить проверки вхождения и непрерывности dynamics

[axisrow]

Closes #47

Summary

• split dynamics continuity and requested-window containment checks into focused methods
• preserve Russian nominative/genitive month parsing and existing compatibility entry point
• clarify containment errors as outside the requested window

Verification

• uv run pytest -q (190 passed)
• uv run ruff check . (clean)
• Valid mutation checks: skipping continuity failed 2 focused tests; skipping containment failed 3; changing the lower-bound boundary failed 4. All mutations were restored before commit.