feat: add as_pint to read_relative() and extract _apply_pint_dtype helper

[CooperBigFoot]

Summary

• Extract inline pint conversion from read() into a reusable _apply_pint_dtype(df, unit) helper (handles empty DataFrames, copies before mutating)
• Add as_pint: bool = False keyword-only parameter to read_relative(), matching the existing read() API
• Document pint-typed array support and IncompatibleUnitError on insert() docstring

Test plan

• Unit tests for _apply_pint_dtype (empty df, applies dtype, no mutation, magnitude preserved)
• Integration tests for read_relative(as_pint=True/False)
• Full test suite passes with no regressions (12 passed, 59 skipped)
• No lint issues in changed files

Closes #8

[FreaxMATE]

Thank you for the PR! Can you please only change code that is related to this PR and drop the bump my version part? Then I can review it again!

[CooperBigFoot]

Cleaned up. Force-pushed with only the issue #8 changes: _apply_pint_dtype helper, as_pint on read_relative(), insert() docstring update, and corresponding tests. Removed version bump, bumpversion config, formatting changes, and lockfile diff.