[browser] CoreCLR in-tree relink

Implements the WASM native re-link pipeline for CoreCLR browser-wasm,
replacing the stub WasmBuildApp / WasmTriggerPublishApp targets with a
full Emscripten-based native build.

Contributes to #123670
Contributes to #126100

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

Author: maraf

eng/native.wasm.targets

@@ -1,14 +1,20 @@
 <Project>
   <Import Project="$(RepositoryEngineeringDir)AcquireEmscriptenSdk.targets" />
 
-  <ItemGroup>
+  <!-- These packages provide ICU and timezone data paths for the native build (CMake).
+       Skip them during app-level relink (IsBrowserWasmProject) since the runtime pack
+       already includes the pre-built native files, and the NuGet contentFiles (timezone
+       data, READMEs) would pollute the app's Content items causing VFS/StaticWebAssets
+       validation failures. -->
+  <ItemGroup Condition="'$(IsBrowserWasmProject)' != 'true'">
     <PackageReference Condition="'$(TargetsWasm)' == 'true'" Include="Microsoft.NETCore.Runtime.ICU.Transport" PrivateAssets="all" Version="$(MicrosoftNETCoreRuntimeICUTransportVersion)" GeneratePathProperty="true" />
     <PackageReference Condition="'$(TargetsWasm)' == 'true'" Include="System.Runtime.TimeZoneData" PrivateAssets="all" Version="$(SystemRuntimeTimeZoneDataVersion)" GeneratePathProperty="true" />
   </ItemGroup>
 
   <PropertyGroup>
     <_RuntimeVariant />
     <_RuntimeVariant Condition="'$(WasmEnableThreads)' == 'true'">-threads</_RuntimeVariant>
+    <FeatureMultithreading Condition="('$(TargetsBrowser)' != 'true' and '$(TargetsWasi)' != 'true') or '$(WasmEnableThreads)' == 'true'">true</FeatureMultithreading>
 
     <_IcuDir Condition="'$(PkgMicrosoft_NETCore_Runtime_ICU_Transport)' != '' and '$(TargetsWasm)' == 'true'">$(PkgMicrosoft_NETCore_Runtime_ICU_Transport)/runtimes/$(TargetOS)-$(TargetArchitecture)$(_RuntimeVariant)/native</_IcuDir>
     <_TzdDir Condition="'$(PkgSystem_Runtime_TimeZoneData)' != '' and '$(TargetsWasm)' == 'true'">$([MSBuild]::NormalizePath('$(PkgSystem_Runtime_TimeZoneData)', 'contentFiles', 'any', 'any', 'data'))</_TzdDir>
@@ -33,18 +39,18 @@
       <NativeCMakeArg Include="-cmakeargs &quot;-DFEATURE_PERFTRACING_DISABLE_PERFTRACING_LISTEN_PORTS=1&quot;" />
       <NativeCMakeArg Include="-cmakeargs &quot;-DFEATURE_PERFTRACING_DISABLE_DEFAULT_LISTEN_PORT=1&quot;" />
   </ItemGroup>
-  <ItemGroup Condition="'$(TargetsBrowser)' == 'true' and '$(RuntimeFlavor)' == 'CoreCLR' and '$(WasmEnableThreads)' != 'true'">
+  <ItemGroup Condition="'$(FeatureMultithreading)' != 'true'">
       <NativeCMakeArg Include="-cmakeargs &quot;-DFEATURE_PERFTRACING_DISABLE_THREADS=1&quot;" />
-      <NativeCMakeArg Include="-cmakeargs &quot;-DFEATURE_SINGLE_THREADED=1&quot;" />
   </ItemGroup>
-  <!-- TODO-WASM - https://github.com/dotnet/runtime/issues/68162
-  <ItemGroup Condition="'$(WasmEnableThreads)' == 'true'">
-    -sUSE_PTHREADS=1
-    -sPTHREAD_POOL_SIZE=0
-    -sPTHREAD_POOL_SIZE_STRICT=0
-    -Wno-pthreads-mem-growth
+  <ItemGroup Condition="'$(FeatureMultithreading)' == 'true'">
+      <NativeCMakeArg Include="-cmakeargs &quot;-DFEATURE_MULTITHREADING=1&quot;" />
+      <!-- TODO-WASM - https://github.com/dotnet/runtime/issues/68162
+        -sUSE_PTHREADS=1
+        -sPTHREAD_POOL_SIZE=0
+        -sPTHREAD_POOL_SIZE_STRICT=0
+        -Wno-pthreads-mem-growth
+      -->
   </ItemGroup>
-  -->
 
   <Target Name="GenerateEmccExports" Condition="'$(TargetsBrowser)' == 'true'">
     <ItemGroup>