fix: remove Windows-specific TFM for Linux/Docker compatibility

Change net8.0-windows to net8.0 in Core, IO, and Engine projects so
the libraries can be consumed in Docker containers running on Linux.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

OpenNest.Core/OpenNest.Core.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net8.0-windows</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <RootNamespace>OpenNest</RootNamespace>
     <AssemblyName>OpenNest.Core</AssemblyName>
   </PropertyGroup>

OpenNest.Engine/OpenNest.Engine.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net8.0-windows</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <RootNamespace>OpenNest</RootNamespace>
     <AssemblyName>OpenNest.Engine</AssemblyName>
   </PropertyGroup>

OpenNest.IO/OpenNest.IO.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net8.0-windows</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <RootNamespace>OpenNest.IO</RootNamespace>
     <AssemblyName>OpenNest.IO</AssemblyName>
   </PropertyGroup>