chore: add from __future__ import annotations to every module so signatures and help() print types as written #635

14 of the package's 55 modules start with from __future__ import annotations. The other 41 don't, so
inspect.signature and help() print their annotations as resolved paths:

CohortAnalysis   (df: pandas.core.frame.DataFrame | ibis.expr.types.relations.Table, aggregation_column: str, ...)
NLRSegmentation  (df: 'pd.DataFrame | ibis.Table', period_col: 'str', ...)

CohortAnalysis is in a module without the import; NLRSegmentation is in one with it and prints the types as the
source writes them.

Change

• Add from __future__ import annotations to every module.
• Make ruff enforce it (rule I002) with required-imports = ["from __future__ import annotations"] under
  [tool.ruff.lint.isort].

Notes

• No module reads annotations at runtime (get_type_hints, __annotations__, pydantic), so nothing depends on them
  being evaluated.
• Each annotation gains quotes ('str'), so the output only gets shorter where it names pandas, ibis or matplotlib
  types. This is a consistency and readability change, not a size change.