Extractive question answering (Xenova/distilbert-base-cased-distilled-squad, grounding corpus context via engine.search topK 1, High/Medium/Low confidence tiers) over its OWN text/PDF corpus, plus Donut (Xenova/donut-base-finetuned-docvqa) document/invoice QA on an uploaded image (~800 MB, gated). CORE engine only — declares no @localmode/langchain or @localmode/wllama. Split from ui/blocks/knowledge-base (Ask tab QA lanes; absorbs qa-bot + invoice-qa).