[CG-18] Test the Elixir extractor on a real codebase

[jgrodziski]

Priority: P2
Owner: Jérémie (hand it to an early Elixir user for field feedback)

Extractor added, not yet exercised. Use a real-world Elixir codebase from an early user to get feedback.

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Source: CodeGraph backlog extracted from the 2026-09-21 weekly (David / Jérémie).