perf(retrieval): tune hybrid recall parameters

[juemimgcd]

Summary

• expand each retrieval backend candidate pool from 50 to 100
• tune the lexical RRF weight from 0.30 to 0.55
• preserve the existing Top-10 output contract and scoped retrieval filters

Evaluation

BEIR SciFact, 300 test queries, 4,000 documents, Recall@10:

• Recall: 0.812222 -> 0.822889 (+0.010667)
• MRR: 0.631085 -> 0.634030 (+0.002945)
• NDCG: 0.668994 -> 0.673534 (+0.004540)

Validation

• Ruff: passed for the changed file
• Existing scoped retrieval tests: 3 passed
• Retrieval package compileall: passed
• git diff --check: passed

No test files were modified.