Enforce return-type annotations via ANN2 ruff rule

[Copilot]

All public methods in src/jquantstats/ already carry explicit return-type annotations. This PR adds lint-time enforcement so future contributions cannot omit them.

Changes

• ruff.toml — adds "ANN2" to extend-select, enabling the full flake8-annotations return-type rule group (ANN201–ANN206) across src/
• ruff.toml per-file-ignores — suppresses ANN for tests/** and **/marimo/** where annotation conventions differ (fixtures, notebook cell functions)
• book/marimo/*.py — -> None: added to cell functions auto-fixed by ruff --fix during the fmt pass

Effect

Any new function added to src/ without a return-type annotation now fails the ruff lint gate:

# ruff: ANN201 Missing return type annotation for public function
def volatility(self, series: pl.Series):   # ← lint error
    ...

# correct
def volatility(self, series: pl.Series) -> float:
    ...

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add return-type annotations to all public methods — Stats methods return float, pl.DataFrame, or pl.Series — all should be explicitly annotated</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add return-type annotations to all public methods — Stats methods return float, pl.DataFrame, or pl.Series — all should be explicitly annotated #383

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