fix: docstring typos and graph_builder None-check bug

[rappdw]

Summary

• Fix "ancester" typo: Corrects "ancester" to "ancestor" in the lookup_relationship docstring
• Fix graph_builder None-check bug: Replaces ambiguous if not self.graph / if self.graph with if self.graph is None / if self.graph is not None — numpy/scipy matrix truthiness is ambiguous and raises errors for non-scalar arrays
• Fix "minumum" typo: Corrects "minumum" to "minimum" in avos_sum docstrings (both in reference and Cython implementations)

Test plan

• Verify pytest tests passes (no behavioral changes expected)
• Confirm the three typo/bug fixes are present in the diff

🤖 Generated with Claude Code

[github-actions]

Package	Line Rate	Complexity	Health
.	68%	0	➖
core	92%	0	✔
reference	98%	0	✔
sparse	94%	0	✔
sparse.csgraph	58%	0	❌
types	100%	0	✔
util	67%	0	➖
Summary	73% (1489 / 2036)	0	➖

Minimum allowed line rate is 65%