chore(rag): add eval knowledge base mirror

knowledge_base/rag-eval/fallback/rag-l0-filter-fallback.md

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+---
+title: RAG L0 Filter Fallback
+keywords: [golden retry contract, second pass retrieval]
+summary: Retry the raw query without the L0 category filter when filtered vector evidence is missing or low quality.
+category: fallback
+source: rag-l0-filter-fallback
+breadcrumb: RAG > Fallback > Unfiltered Retry
+kb_scope: rag-eval
+covers: [fallback, unfiltered retry, retrieval quality]
+when_to_retrieve: Use when validating the fallback contract for low-quality filtered vector retrieval.
+---
+
+# RAG
+
+## Fallback
+
+### Unfiltered Retry
+
+If the first vector search is over-constrained by an L0 metadata filter and returns low quality evidence,
+the retriever should skip the L0 filter and run an unfiltered vector retry with the original query.
+
+The fallback reason should be `filtered_vector_low_quality` when the filtered candidate exists but is below the
+reference threshold. If there is no usable evidence at all, use `filtered_vector_no_evidence`.
+
+This document is the expected evidence for skip the L0 filter, unfiltered vector retry, and low quality behavior.