Non-web projects

A console app, or a NativeAOT binary that serves its own UI, has no static web assets. Embed the output in the assembly instead.

<PropertyGroup>
  <PareEmbed>true</PareEmbed>
  <PareEmbedPrefix>ui/</PareEmbedPrefix>
  <EnablePareDefaultItems>false</EnablePareDefaultItems>
</PropertyGroup>
<ItemGroup>
  <PackageReference Include="Extrode.Pare.Build" Version="0.6.2" PrivateAssets="all" />
  <TypeScriptSource Include="web/**/*.ts" />
</ItemGroup>

web/lib/util.ts is written to obj/<configuration>/<tfm>/pare/web/lib/util.js, never into the source tree, and embedded as ui/web/lib/util.js. Read it back:

using var stream = typeof(Program).Assembly.GetManifestResourceStream("ui/web/lib/util.js");

• The resource name is the prefix plus the path below the project. PareServeAt changes the path: <TypeScriptSource Include="web/**/*.ts" PareServeAt="/" /> gives ui/lib/util.js.
• The outputs are not Content, not static web assets, and not copied to the build or publish output.
• A name such as strings.de.ts stays in the main assembly (no satellite assembly for de).
• The step is incremental: a .ts edit rewrites its .js and recompiles; a deleted .ts drops its resource and its .js on the next build.

Do it your own way

To embed or copy the files differently, leave PareEmbed off and use the @(PareOutput) item:

<PropertyGroup>
  <PareOutDir>$(BaseIntermediateOutputPath)pare/</PareOutDir>
</PropertyGroup>
<Target Name="EmbedPare" AfterTargets="PareStripTypeScript">
  <ItemGroup>
    <EmbeddedResource Include="@(PareOutput)" LogicalName="ui/%(PareOutput.PareRelativePath)" />
  </ItemGroup>
</Target>

$(IntermediateOutputPath) is still empty where a project file sets its properties, so PareOutDir cannot use it. $(BaseIntermediateOutputPath) (obj/) is set by then.

After PareStripTypeScript, @(PareOutput) lists every output. %(PareSource) is the source's full path. %(PareRelativePath) uses /: the URL path for a static web asset (js/app.js), the resource name without the prefix under PareEmbed, and otherwise the output's path below the project (or below PareOutDir). A target with AfterTargets="PareStripTypeScript" sees it on every build, including builds that skip the strip as up to date.