Concept-Check: Retrieval

You have installed the fourth and final seam this arc: the retrieval split — structured filters for the numbers, embeddings for the prose — behind the Embedder trait and a brute-force cosine VectorIndex. This check pins down the parts that are easy to get almost right: which field is meaning-bearing prose (and why the numbers stay out of the index), why embedding is the definitive content-addressed workload, how the seen/content_id cache guarantees a text is embedded at most once, and the exact behavior of cosine and top_k.

Three of these are Tracing questions: read the program, decide whether it compiles, and if it does, predict its exact output before revealing the answer. Predicting is the point — the cosine and top_k traces are the same arithmetic the shipped index runs.