Port to .NET Framework 4.8 with C# 9.0 compatibility

- Change target framework from net6.0 to net48 with LangVersion 9.0
- Remove nullable reference types for .NET Framework compatibility
- Replace Convert.FromHexString/ToHexString with custom implementations
- Fix switch pattern matching to use traditional case statements
- Replace range operators with LINQ Take/Skip and Array.Copy
- Replace Math.Log2 with Math.Log(p)/Math.Log(2) formula
- Fix Contains method calls to use ToLower() instead of StringComparison

🤖 Generated with [Claude Code](https://claude.ai/code)

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

V2GProtocol.csproj

@@ -2,9 +2,8 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net6.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
+    <TargetFramework>net48</TargetFramework>
+    <LangVersion>9.0</LangVersion>
     <AssemblyName>V2GDecoder</AssemblyName>
   </PropertyGroup>