Implement CoreCLR browser-wasm native build targets

Replace placeholder 'not implemented' targets in BrowserWasmApp.CoreCLR.targets
with a working implementation that copies pre-built native files from the runtime
pack. Unlike Mono which re-links dotnet.native.wasm per-app via emcc, CoreCLR
pre-links it once during the runtime build (corehost.proj).

Changes:
- Import WasmApp.Common.props/targets for shared WasmBuildApp chain
- Set WasmBuildNative=false to skip per-app emcc compile/link
- Override _SetupToolchain to mark emscripten as unavailable (no src/ dir in
  CoreCLR runtime pack)
- Add _CoreCLRBrowserCopyNativeAssets target to copy dotnet.native.wasm/js from
  runtime pack and register as WasmNativeAsset items for SDK bundling
- Add DotNetCoreWeb ProjectCapability for VS debugging support

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

Author: maraf

src/mono/browser/build/BrowserWasmApp.CoreCLR.targets

@@ -1,9 +1,55 @@
 <Project>
-  <!-- TODO-WASM: These targets are invoked by public WasmSDK -->
-  <Target Name="WasmBuildApp">
-    <Message Importance="high" Text="WasmBuildApp for CoreCLR not implemented" />
+  <!--
+    CoreCLR browser-wasm app build targets.
+
+    Unlike Mono, CoreCLR pre-links dotnet.native.wasm once during the runtime build (via corehost.proj).
+    The pre-built native files (dotnet.native.wasm, dotnet.native.js, etc.) are already in the runtime pack.
+    This targets file imports the shared WasmApp.Common.targets infrastructure but skips per-app emcc
+    compile/link by setting WasmBuildNative=false and marking the Emscripten toolchain as unavailable.
+  -->
+
+  <Import Project="$(WasmCommonTargetsPath)WasmApp.Common.targets" />
+
+  <ItemGroup>
+    <!-- Allow running/debugging from VS -->
+    <ProjectCapability Include="DotNetCoreWeb" />
+  </ItemGroup>
+
+  <!--
+    Override _SetupToolchain to mark the Emscripten toolchain as missing.
+    CoreCLR does not need emcc for app builds since native linking is done at runtime build time.
+    This skips _ReadWasmProps (which reads wasm-props.json from a src/ dir that doesn't exist for CoreCLR)
+    and all emcc-related targets.
+  -->
+  <Target Name="_SetupToolchain">
+    <PropertyGroup>
+      <_IsToolchainMissing>true</_IsToolchainMissing>
+    </PropertyGroup>
   </Target>
-  <Target Name="WasmTriggerPublishApp">
-    <Message Importance="high" Text="WasmTriggerPublishApp for CoreCLR not implemented" />
+
+  <!--
+    Copy pre-built native files from the runtime pack to the intermediate output path,
+    and register them as WasmNativeAsset items so the WebAssembly SDK can include them
+    in the app bundle (boot JSON, static web assets, etc.).
+  -->
+  <Target Name="_CoreCLRBrowserCopyNativeAssets"
+          AfterTargets="PrepareInputsForWasmBuild"
+          Condition="'$(MicrosoftNetCoreAppRuntimePackRidNativeDir)' != ''">
+    <ItemGroup>
+      <_CoreCLRNativeAsset Include="$(MicrosoftNetCoreAppRuntimePackRidNativeDir)dotnet.native.wasm" />
+      <_CoreCLRNativeAsset Include="$(MicrosoftNetCoreAppRuntimePackRidNativeDir)dotnet.native.js" />
+      <_CoreCLRNativeAsset Include="$(MicrosoftNetCoreAppRuntimePackRidNativeDir)dotnet.native.js.symbols"
+                           Condition="Exists('$(MicrosoftNetCoreAppRuntimePackRidNativeDir)dotnet.native.js.symbols')" />
+      <_CoreCLRNativeAsset Include="$(MicrosoftNetCoreAppRuntimePackRidNativeDir)dotnet.native.worker.mjs"
+                           Condition="Exists('$(MicrosoftNetCoreAppRuntimePackRidNativeDir)dotnet.native.worker.mjs')" />
+    </ItemGroup>
+
+    <Copy SourceFiles="@(_CoreCLRNativeAsset)"
+          DestinationFolder="$(_WasmIntermediateOutputPath)"
+          SkipUnchangedFiles="true" />
+
+    <ItemGroup>
+      <WasmNativeAsset Include="@(_CoreCLRNativeAsset->'$(_WasmIntermediateOutputPath)%(FileName)%(Extension)')" />
+    </ItemGroup>
   </Target>
-</Project>
+</Project>

src/mono/browser/build/WasmApp.InTree.props

@@ -14,10 +14,14 @@
   <Import Sdk="Microsoft.NET.Sdk.WebAssembly" Project="Sdk.props" Condition="'$(UsingNativeAOT)' != 'true' and '$(UsingMicrosoftNETSdkWebAssembly)' == 'true'" />
   <Import Project="$(MSBuildThisFileDirectory)BrowserWasmApp.props" Condition="'$(UsingNativeAOT)' != 'true' and '$(RuntimeFlavor)' == 'Mono'" />
 
-  <!-- TODO-WASM https://github.com/dotnet/runtime/issues/120248 (CoreCLR on wasm defaults) -->
+  <!-- CoreCLR on browser-wasm: import common props for shared WasmBuildApp chain definitions -->
+  <Import Project="$(WasmCommonTargetsPath)WasmApp.Common.props" Condition="'$(UsingNativeAOT)' != 'true' and '$(RuntimeFlavor)' == 'CoreCLR'" />
   <PropertyGroup Condition="'$(RuntimeFlavor)' == 'CoreCLR'">
     <InvariantGlobalization Condition="'$(InvariantGlobalization)' == ''">false</InvariantGlobalization>
     <WasmEnableWebcil>false</WasmEnableWebcil>
+    <!-- CoreCLR pre-links dotnet.native.wasm during the runtime build (corehost.proj).
+         No per-app emcc compile/link is needed. -->
+    <WasmBuildNative>false</WasmBuildNative>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(IsTestProject)' == 'true' or '$(IsOutputTypeLibrary)' == 'true'">