Projektdateien hinzufügen.

FIXES_NET48_DUAL_TARGET.md

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+# FIX2 - .NET Framework 4.8 dual target
+
+Changes in this package:
+
+1. `IMSApiDotNet.csproj` now uses:
+   - `netstandard2.0`
+   - `net48`
+2. Added private build-only reference:
+   - `Microsoft.NETFramework.ReferenceAssemblies.net48` 1.0.3
+3. Added dedicated build helpers:
+   - `scripts/build-net48.ps1`
+   - `scripts/build-net48.cmd`
+4. Added `NET48_BUILD.md`.
+5. Kept the protocol-vector test project on `net7.0` so it works with the user's installed .NET SDK 7.0.302.
+6. Kept the previous FIX1 compiler corrections in `IhapReader.cs` and `WireRegistry.cs`.
+
+Expected Release outputs:
+
+- `src/IMSApiDotNet/bin/Release/netstandard2.0/IMSApiDotNet.dll`
+- `src/IMSApiDotNet/bin/Release/net48/IMSApiDotNet.dll`
+
+Use the `net48` DLL in classic .NET Framework 4.8 applications.