Fix duplicate StaticWebAsset Identity crash for multi-WASM-client solutions

PR #124125 changed WASM ContentRoot to per-item %(RootDir)%(Directory), which
makes each asset's Identity its real file path on disk. This is correct for
single-client scenarios and fixes SRI integrity on incremental builds, but
breaks multi-WASM-client solutions: NuGet cache pass-through files (JS, maps,
ICU, native wasm) resolve to the same Identity across projects, causing a
ToDictionary crash in DiscoverPrecompressedAssets.

The fix copies pass-through files from the shared NuGet cache to the per-project
$(IntermediateOutputPath)webcil/ directory (where webcil-converted files already
live), so every project gets a unique Identity. Satellite assemblies are placed
in culture subdirectories to match ConvertDllsToWebcil behavior.

This follows the pattern from sdk#52816 and Javier's SWA design principle that
Identity should match a real file on disk.

Fixes the aspnetcore codeflow regression where solutions hosting multiple Blazor
WASM clients fail with:
  System.ArgumentException: An item with the same key has already been added.
  Key: ...dotnet.js.map
  at DiscoverPrecompressedAssets.Execute()

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

Author: lewing

src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets

@@ -360,7 +360,39 @@ Copyright (c) .NET Foundation. All rights reserved.
       <Output TaskParameter="FileWrites" ItemName="FileWrites" />
     </ConvertDllsToWebcil>
 
+    <!-- Copy pass-through files (JS, maps, ICU, native wasm, and when WebCil is disabled, DLLs)
+         from shared locations (e.g. NuGet cache) to a per-project directory so each project gets
+         a unique Identity when multiple WASM client projects reference the same runtime pack.
+         Webcil-converted files already live in per-project $(IntermediateOutputPath)webcil/ and
+         don't need copying. Satellite assemblies are placed in culture subdirectories to match
+         ConvertDllsToWebcil behavior. -->
     <ItemGroup>
+      <_WasmPassThroughCandidates Include="@(_WebcilAssetsCandidates)"
+        Condition="!$([System.String]::new('%(Identity)').StartsWith($(_WasmBuildWebcilPath)))" />
+      <_WebcilAssetsCandidates Remove="@(_WasmPassThroughCandidates)" />
+    </ItemGroup>
+
+    <!-- Separate satellite (culture) pass-throughs from regular ones so we can place them in subdirs -->
+    <ItemGroup>
+      <_WasmCulturePassThroughs Include="@(_WasmPassThroughCandidates)"
+        Condition="'%(_WasmPassThroughCandidates.AssetTraitName)' == 'Culture'" />
+      <_WasmPassThroughCandidates Remove="@(_WasmCulturePassThroughs)" />
+    </ItemGroup>
+
+    <Copy SourceFiles="@(_WasmPassThroughCandidates)"
+      DestinationFolder="$(_WasmBuildWebcilPath)"
+      SkipUnchangedFiles="true" />
+
+    <Copy SourceFiles="@(_WasmCulturePassThroughs)"
+      DestinationFiles="@(_WasmCulturePassThroughs->'$(_WasmBuildWebcilPath)%(AssetTraitValue)/%(Filename)%(Extension)')"
+      SkipUnchangedFiles="true"
+      Condition="'@(_WasmCulturePassThroughs)' != ''" />
+
+    <ItemGroup>
+      <FileWrites Include="@(_WasmPassThroughCandidates->'$(_WasmBuildWebcilPath)%(Filename)%(Extension)')" />
+      <FileWrites Include="@(_WasmCulturePassThroughs->'$(_WasmBuildWebcilPath)%(AssetTraitValue)/%(Filename)%(Extension)')" />
+      <_WebcilAssetsCandidates Include="@(_WasmPassThroughCandidates->'$(_WasmBuildWebcilPath)%(Filename)%(Extension)')" />
+      <_WebcilAssetsCandidates Include="@(_WasmCulturePassThroughs->'$(_WasmBuildWebcilPath)%(AssetTraitValue)/%(Filename)%(Extension)')" />
       <!-- Set per-item ContentRoot so each asset's Identity matches its actual file on disk -->
       <_WebcilAssetsCandidates Update="@(_WebcilAssetsCandidates)" ContentRoot="%(RootDir)%(Directory)" />
       <_WasmFingerprintPatterns Include="WasmFiles" Pattern="*.wasm" Expression="#[.{fingerprint}]!" />