SCPCResult summary methods

[DGoettlich]

Summary

Adds R-like result helpers for SCPCResult in scpc-python>=0.1.2.

This PR makes SCPC results easier to inspect by storing coefficient names and implementing print(result), summary(), coef(), and confint(). It also adds OLS and IV examples, updates docs to prefer print(result) in examples, and keeps raw scpcstats / scpccvs available for lower-level access.

It also narrows the pyfixest IV design return type with FixestSpec, which fixes ty diagnostics without changing runtime behavior.

There are no numerical changes.

Testing

• uv run -m pytest
• uv run ruff check .
• uv run ty check tests

Notes

• The docs mention that the new R-like helpers are available from scpc-python>=0.1.2.