Fix documentation and data integrity safeguards

[dshkol]

Summary

• fix Sphinx-Gallery thumbnail links and restore geographic example plots
• make get_census() cache keys canonical, schema-versioned, and sensitive to geometry resolution and label mode
• preserve CensusAPIError subclasses across public API entry points and add NetworkError for exhausted connection failures
• require complete, unique geographic identifiers when combining geometry and vector responses

Root causes

Generated notebooks were being discovered as the canonical Sphinx documents, which left the gallery's RST reference targets unresolved. The geographic example also used NumPy without importing it.

The census cache key omitted output-affecting options, public wrappers replaced resilience exceptions with generic RuntimeError instances, and geometry/vector responses fell back to positional alignment when identifiers were unavailable.

Impact

Gallery thumbnails now open their example pages and the geographic plots execute correctly. Cached high-resolution and label-mode variants cannot collide. Callers can catch the documented API exception hierarchy, and ambiguous geography/vector combinations fail explicitly instead of returning potentially misaligned census data.

Validation

• 114 deterministic tests passed
• black --check passed
• critical flake8 checks passed
• git diff --check passed
• full Sphinx HTML build completed with all gallery examples executed

Closes #24

[github-actions]

🧪 R Equivalence Validation Results

VALIDATION SUMMARY
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See full validation report in artifacts.