Freeze mixed-file retrieval diagnostic and shared-engine evaluator

fixtures/mixed/README.md

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+# Mixed All-files diagnostic v1
+
+This is a frozen, small diagnostic, **not a production quality score**. No mixed
+model run has been performed yet.
+
+The corpus contains 32 actual files: six searchi implementations, four searchi
+technical documents, two complete pinned llama.cpp source files, and twenty
+existing licensed media files. Source snapshots retain exact original bytes;
+`fixture.json` records upstream paths, commits, SHA-256, sizes, and licenses.
+The llama.cpp copies come from committed upstream objects, excluding local
+runtime patches. Its MIT notice is in `LLAMA-LICENSE.txt`. Media files are
+referenced from `../media/assets` rather than duplicated. Their source URLs,
+licenses, independently observed labels and attribution remain in
+[the media fixture](../media/README.md).
+
+There are 91 frozen queries: English/German implementation and document pairs,
+65 reused media queries, four additional absent-content queries and four pinned
+upstream implementation queries. Code queries run in both All files and Code
+scope. Documents about the implementation coexist with the code, providing
+realistic competing content. Speech, music, images and videos compete with
+both. The case asking for code that chooses speech search is intentional: it
+can expose audio-intent routing that suppresses a relevant implementation.
+
+Labels were written by one annotator before **mixed** inference. Existing media
+results had already been observed in their separate evaluation, so this is not
+an untouched held-out media benchmark. searchi sources are the application's
+own repository; the two llama.cpp files add external code, but do not establish
+quality across unrelated real projects. Never adjust v1 labels after seeing
+mixed results. Corrections require a new fixture version, explicit reasoning,
+and a new pre-inference lock. A second blinded human review remains open.
+
+Run `python tools/mixed-eval/evaluate.py --verify-only` to verify labels and all
+source/media bytes without invoking a model. See
+[the evaluator instructions](../../tools/mixed-eval/README.md) for real runs.