chore(deps): loosen optional dependency lower bounds

[w-martin]

Summary

• Fixes chore(deps): loosen optional dependency lower bounds #8
• Audited src/typedframes/pandas.py, polars.py, pandera.py for minimum API requirements

Rationale

• pandas: >=2.3.3 -> >=2.0 — only uses pd.DataFrame subclassing/_metadata/constructor/.groupby()/pd.read* — all stable since 2.0
• pandera: >=0.29.0 -> >=0.20.0 — only uses pa.DataFrameSchema and pa.Column(dtype, nullable, regex)
• polars: >=1.38.1 -> >=1.0 — only uses pl.read_*/pl.DataFrame as type annotation; 1.0 (Jul 2024) stabilised the API

Test plan

• pyproject.toml is valid TOML
• Dev deps in uv.lock are unaffected (optional extras only)

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