feat(eng): enable package validation in fsproj (#259)

Add EnablePackageValidation=true to the library project. This activates
the .NET SDK's built-in package validation on every build, catching
package structure problems early (e.g. missing frameworks, mismatched
assemblies across TFMs).

A commented-out PackageValidationBaselineVersion property is included as
a guide for enforcing binary-compatibility checks against a previously-
published version. Uncomment and set to e.g. '0.4.0' (or the most
recently published version) to have the build fail if any public API is
removed or signature-changed.

Build verified: 0 warnings, 0 errors.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

src/FSharp.Control.TaskSeq/FSharp.Control.TaskSeq.fsproj

@@ -25,6 +25,10 @@ Generates optimized IL code through resumable state machines, and comes with a c
     <PackageTags>taskseq;f#;fsharp;asyncseq;seq;sequences;sequential;threading;computation expression;IAsyncEnumerable;task;async;iteration</PackageTags>
     <IncludeSymbols>True</IncludeSymbols>
     <SymbolPackageFormat>snupkg</SymbolPackageFormat>
+    <!-- Validate package structure and (when PackageValidationBaselineVersion is set) binary compatibility -->
+    <EnablePackageValidation>true</EnablePackageValidation>
+    <!-- Set this to the last published version to enforce binary compatibility, e.g. 0.4.0 -->
+    <!-- <PackageValidationBaselineVersion>0.4.0</PackageValidationBaselineVersion> -->
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">