docs(readme): 更新为实测层次性能对比 + standalone 存储 WAL 耐久性

[NeverENG]

更新 README 使其与当前实现一致，并补上本轮性能优化的实测数据。

修正过时内容

• 删除"存储层自身无 WAL""崩溃恢复均由 Raft 负责"等已不成立的描述。
• 架构图/说明补齐：双入口（BanNet + gRPC）汇聚到同一 KVServer；standalone 存储层 WAL（group commit + checkpoint 自清洁）；SSTable 重启重载恢复。

新增「性能：各层开销实测对比」

路径	并发	QPS	P50
GET（纯内存）	50	16,806	2.45ms
PUT 改前（每写 fsync）	50	225	203ms
PUT group commit	50	6,087	8ms
PUT group commit	200	22,231	8ms

量化了写路径瓶颈 100% 在 fsync（225≈本机单次 fsync 上限），以及 group commit 让吞吐随并发摊销。

其他

核心能力补背压/边缘查询/可观测性；路线图勾掉已完成项。

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Reviewing files that changed from the base of the PR and between 51d82b0 and 2820a02.

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• README.md

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• Commit unit tests in branch docs/readme-perf-durability

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[github-actions]

🐯 BanGD 数据库内核评审

整体风险：🟢 低

变更总结：这是一个纯文档（README.md）更新 PR，目的是使文档与当前实现一致。核心变更包括：1) 删除过时描述（"存储层自身无 WAL""崩溃恢复均由 Raft 负责"）——反映出 standalone 模式已添加独立存储层 WAL（group commit + checkpoint 自清洁）与崩溃恢复；2) 架构图补齐双入口（BanNet + gRPC → 同一 KVServer）、standalone 写路径经存储层 WAL、SSTable 重启重载；3) 新增端到端压测数据表，量化 group commit 的性价比（每写 fsync 225 qps → group commit 22k qps）；4) 核心能力清单补上背压、边缘范围查询、可观测性；5) 路线图勾掉已完成项。不涉及任何代码变更。

本评审不阻塞合入；架构级建议以 Issue 形式跟踪，普通问题在下方内联列出。

未发现需要从架构层面改进的问题。

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_{本次评审消耗 token：共 16690 tokens（输入 10109，输出 309，缓存命中 6272，缓存写入 0）｜维度 [concurrency, memory, lock, storage, performance]｜对抗式复核 3 票/条，过滤疑似误报 0 条}