Fix collapsed import self-loops

[Endogen]

Summary

• drop file-level imports, imports_from, and re_exports edges when endpoint resolution collapses them into self-loops
• cover Python stdlib/package basename collisions and Rust parent/external module cases
• verify that meaningful recursive calls self-loops remain intact

Fixes #2037.

Root cause

AST extraction can emit unresolved bare import targets such as builtins, contracting, or poseidon. The legacy-ID alias index in build_from_json() may then resolve that bare target to the importing file's own canonical node. Materializing the resolved edge creates a file-level self-loop that carries no connectivity and is flagged by graphify diagnose multigraph.

The filter is intentionally relation-specific. It removes only collapsed import/re-export edges and preserves other self-edges, including recursive calls.

Impact

Rebuilt graphs no longer contain false import self-loops caused by basename/legacy-alias resolution. This eliminates diagnostic noise without changing application code or hiding real recursive program structure.

Validation

• uv run pytest tests/ -q — 3,478 passed, 3 skipped
• uv run ruff check . — passed
• focused build/extraction suite — 148 passed
• rebuilt two affected real-world graphs — zero self-loops, dangling endpoints, missing endpoints, or endpoint-collapse groups