fix(kb): make paper search evidence-safe

[GTC2333]

Summary

• return a structured paper-search status envelope and distinguish recoverable misses from infrastructure failures
• normalize title and phrase matching without erasing semantic symbols, while keeping exact matches first
• keep metadata search metadata-only and preserve full-text recall in full-paper mode
• bound topk, report missing/unreadable text and unavailable pages, and prevent zero-hit keyword-only results
• document the updated English and Chinese contracts

Compatibility

• searchPapers is internal to Runtime and has no public package export or additional direct callers
• search_papers_local continues to return JSON text, now as an explicit object with status and results

Validation

• Runtime build: passed
• Runtime typecheck: passed
• Focused search/tool tests: passed
• Full test suite: 107 files, 1133 tests passed