build: disable StaticWebAssets cache to stop MSB4018 race

DefineStaticWebAssets.ReadOrCreateCache opens the *.dswa.cache.json
files under obj/.../ via File.OpenRead with no try/catch, so under
parallel solution builds a sharing-violation / TOCTOU race on the cache
file escapes as MSB4018 and fails the whole build (seen on
Yavsc.Blogs during the rc15 solution build).

Set StaticWebAssetsCacheDefineStaticWebAssetsEnabled=false so
CacheManifestPath is empty and ReadOrCreateCache short-circuits to a
no-file-I/O path. The cache is a pure build-speed optimization; no
effect on output correctness, only on incremental rebuild speed of the
wwwroot-light web projects.

Verified: Yavsc.Api (transitive web deps) and Yavsc.Blogs.Tests build
clean, and zero *.dswa.cache.json files are produced.

Refs:
  https://source.dot.net/Microsoft.NET.Sdk.StaticWebAssets.Tasks/DefineStaticWebAssets.Cache.cs.html
  https://github.com/dotnet/sdk/issues/49039

Co-Authored-By: Claude <noreply@anthropic.com>

Directory.Build.props

@@ -2,5 +2,16 @@
   <PropertyGroup>
     <RootNamespace>Yavsc</RootNamespace>
     <NoWarn>NU1701, NU1901, NU1902, NU1507</NoWarn>
+    <!-- Disable the StaticWebAssets build cache (the *.dswa.cache.json
+         files written under obj/.../ by DefineStaticWebAssets).
+         ReadOrCreateCache opens them with File.OpenRead and no
+         try/catch, so under parallel solution builds a sharing
+         violation / TOCTOU race on the cache file escapes as
+         MSB4018 and fails the whole build (seen on Yavsc.Blogs
+         during the rc15 solution build). The cache is a pure
+         build-speed optimization; turning it off has no effect on
+         output correctness, only on incremental rebuild speed of
+         the (wwwroot-light) web projects. -->
+    <StaticWebAssetsCacheDefineStaticWebAssetsEnabled>false</StaticWebAssetsCacheDefineStaticWebAssetsEnabled>
   </PropertyGroup>
 </Project>