feat: add final LLM filter stage (文献器) after rerank

[uf-hy]

What

• Add optional final-stage LLM filter/extractor ("文献器") after cross-encoder rerank to select+extract relevant content, then surface extracted items first.
• Increase default QMD_RERANK_DOC_LIMIT in QMD_RERANK_MODE=rerank to 120 (keep 40 for LLM mode).
• Clamp QMD_CHUNK_OVERLAP_TOKENS to avoid zero/negative step (hang risk).

How to enable

Set QMD_LLM_FILTER_DOC_LIMIT to a positive number (e.g. 40) and configure provider via existing QMD_OPENAI_* or QMD_GEMINI_* (or QMD_LLM_FILTER_PROVIDER).

Notes

• Keeps non-extracted results as fallback (extracted items are moved to the top).
• Tests: bun test.