fix(managed): distinguish .NET Standard targets from runtimes

Preserve .NET Standard as an API compatibility target while leaving the runtime family unknown unless independent markers identify it. Keep real DLL metadata and existing runtime-marker precedence intact.

Closes #1736.

Co-authored-by: AC <ac@fabtek.cn>

docs/managed-code-analysis.md

@@ -38,6 +38,12 @@ Inspect separately obtained components explicitly and preserve their identities.
 A valid implementation marker establishes a candidate native boundary, not
 recovered native semantics or runtime behavior.
 
+An assembly targeting [.NET Standard](https://learn.microsoft.com/en-us/dotnet/standard/net-standard)
+declares API compatibility across multiple .NET implementations. REA preserves
+that target in `target_frameworks` and leaves `runtime_family` as `unknown`
+unless independent markers identify a runtime family, such as a ReadyToRun
+signature.
+
 ### Resource admission
 
 The static provider admits regular input files up to 128 MiB before retaining