Update 系统设计基础.md

补充熔断器的三种状态及状态转换图。

tools/diagram/src/circuit-breaker.svg

@@ -0,0 +1,47 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="520" height="270" viewBox="0 0 520 270">
+  <style>
+    text { font-family: "PingFang SC", "Microsoft YaHei", Helvetica, Arial, sans-serif; fill: #666; font-size: 12px; }
+    .title { font-weight: bold; font-size: 15px; fill: #777; text-anchor: middle; }
+    .state { font-weight: bold; font-size: 14px; text-anchor: middle; }
+    .en { font-size: 10px; fill: #888; text-anchor: middle; }
+    .arrow { stroke: #666; stroke-width: 1.3; fill: none; marker-end: url(#a); }
+    .shadow { fill: #C0C0C0; }
+  </style>
+  <defs>
+    <marker id="a" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto">
+      <path d="M1,1 L9,5 L1,9" fill="none" stroke="#666" stroke-width="1.6"/>
+    </marker>
+  </defs>
+  <rect width="100%" height="100%" fill="#FFFFFF"/>
+  <text x="260" y="28" class="title">熔断器状态</text>
+
+  <!-- 关闭：正常调用 -->
+  <rect x="58" y="81" width="110" height="44" rx="6" class="shadow"/>
+  <rect x="55" y="78" width="110" height="44" rx="6" fill="#B0E3E6"/>
+  <text x="110" y="98" class="state">关闭</text>
+  <text x="110" y="114" class="en">Closed，正常调用</text>
+
+  <!-- 打开：直接返回默认结果 -->
+  <rect x="358" y="81" width="110" height="44" rx="6" class="shadow"/>
+  <rect x="355" y="78" width="110" height="44" rx="6" fill="#F3C6C2"/>
+  <text x="410" y="98" class="state">打开</text>
+  <text x="410" y="114" class="en">Open，直接返回</text>
+
+  <!-- 半开：放行少量试探请求 -->
+  <rect x="208" y="193" width="110" height="44" rx="6" class="shadow"/>
+  <rect x="205" y="190" width="110" height="44" rx="6" fill="#B1DDF0"/>
+  <text x="260" y="210" class="state">半开</text>
+  <text x="260" y="226" class="en">Half-Open，少量试探</text>
+
+  <line x1="169" y1="100" x2="351" y2="100" class="arrow"/>
+  <text x="260" y="92" text-anchor="middle">失败率超过阈值</text>
+
+  <line x1="378" y1="125" x2="297" y2="187" class="arrow"/>
+  <text x="318" y="145" text-anchor="end">等待一段时间</text>
+
+  <line x1="310" y1="190" x2="391" y2="128" class="arrow"/>
+  <text x="362" y="176">试探请求失败</text>
+
+  <line x1="218" y1="190" x2="137" y2="128" class="arrow"/>
+  <text x="160" y="176" text-anchor="end">试探请求成功</text>
+</svg>