Function estimate_lookback_days 

fn estimate_lookback_days(kl_type: KLType, count: i32) -> i32

给定 kl_type 和要求根数，估算回溯天数。

eli BUG-006 fix (P2, 2026-04-27): 之前 Day padding +10 导致 lookback 比 N 大太多, backend cmd 1100 返 first N 落在 range 最前 而非 最近. eli 真机 verify: count=5 today=04-27 backend 返 04-09 to 04-15 (oldest 5). 用户期望 “last N trading days” → daemon 应紧贴 N tradedays 让 backend first N ≈ newest N.

修法: lookback_days = ceil(N * 7/5) + 3 (5 天/周转换 + 假日 buffer).

• count=5 → 10 days. Range [today-10, today] = 5-7 trading days. backend first 5 = newest 5. ✅ (vs 旧版 18 天 = 12-13 trading days, first 5 = oldest 5).
• count=100 → 143 days ≈ 100+ trading days. backend first 100 ≈ newest 100.