feat: 统一服务缓存 + 指纹驱动插件匹配

将 webServiceCache 扩展为通用 serviceCache，所有指纹识别结果
统一缓存，插件匹配时端口不命中则回退到服务名称匹配。

删除多余的 service_cache.go，复用已有的 ServiceInfo 体系。
补充 nil 防御、Explicit 标记、大量单元/集成/回归测试。

webscan/lib/client_test.go

@@ -0,0 +1,24 @@
+package lib
+
+import "testing"
+
+func TestNormalizeHTTPProxyURL(t *testing.T) {
+	tests := []struct {
+		name string
+		in   string
+		want string
+	}{
+		{name: "port shortcut", in: "8080", want: "http://127.0.0.1:8080"},
+		{name: "ipv4 host port", in: "127.0.0.1:8080", want: "http://127.0.0.1:8080"},
+		{name: "hostname port", in: "proxy.local:8080", want: "http://proxy.local:8080"},
+		{name: "bracketed ipv6 port", in: "[::1]:8080", want: "http://[::1]:8080"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := normalizeHTTPProxyURL(tt.in); got != tt.want {
+				t.Fatalf("normalizeHTTPProxyURL(%q) = %q, want %q", tt.in, got, tt.want)
+			}
+		})
+	}
+}