feat: Snapshot-Offset 限额策略替换 BUFFER 盲猜

核心改动:不再用 BUFFER 猜滞后窗口消耗,而是用同步时点快照
 直接计算累计限额。

- BUFFER=300000 → SMALL_BUFFER=50000(仅覆盖非消耗性跳降)
- get_effective_limit: limit = B0 - SMALL_BUFFER,不依赖 current_usage
- 同步循环:写 balance_at_sync + usage_at_sync 替代 remaining/stable_limit
- 突发检测/低额度判断:读 balance_at_sync 替代 remaining
- volc_consume.py: vls.BUFFER → vls.SMALL_BUFFER
- debug/debug_volc_sync.py: 同步更新
- docs/volc-quota-notes.md: 新增 §7.6 v6 设计文档,更新各节口径

debug/debug_volc_sync.py

@@ -20,7 +20,7 @@ SECRET_KEY = os.getenv(
 )
 HOST = "open.volcengineapi.com"
 REGION = "cn-beijing"
-BUFFER = 300000
+SMALL_BUFFER = 50000
 
 # Suffixes to clean from Volcengine response
 SUFFIXES_TO_REMOVE = [
@@ -203,10 +203,10 @@ if __name__ == "__main__":
 
     print("\n--- Final Model Balances ---")
     for model, balance in model_balances.items():
-        remaining = balance - BUFFER
+        remaining = balance - SMALL_BUFFER
         print(f"Model: {model}")
         print(f"  Balance  : {balance:,}")
-        print(f"  Buffer   : {BUFFER:,}")
+        print(f"  SmallBuffer: {SMALL_BUFFER:,}")
         print(f"  Remaining: {remaining:,}")
 
         # Simulate Redis Keys