Build command can't find output file in subdirectory with custom output path

Description

When a SpacetimeDB C# module is:

1. Located in a subdirectory (part of a multi-app repo)
2. Using root-level Directory.Build.props to customize output paths

The spacetime build command reports success but fails with "Built project successfully but couldn't find the output file".

Steps to Reproduce

1. Create a multi-app directory structure:

my-project/
  apps/
    api/  # SpacetimeDB module here
    other-app/
  dist/
  Directory.Build.props

2. Add Directory.Build.props in root with dynamic output paths:

<Project>
  <PropertyGroup>
    <RepoRoot>$([System.IO.Path]::GetFullPath('$(MSBuildThisFileDirectory)'))</RepoRoot>
    <ProjectRelativePath>$([MSBuild]::MakeRelative($(RepoRoot), $(MSBuildProjectDirectory)))</ProjectRelativePath>
    <BaseOutputPath>$(RepoRoot)dist/$(ProjectRelativePath)</BaseOutputPath>
    <OutputPath>$(BaseOutputPath)</OutputPath>
    <BaseIntermediateOutputPath>$(RepoRoot)dist/intermediates/$(ProjectRelativePath)/obj</BaseIntermediateOutputPath>
    <IntermediateOutputPath>$(BaseIntermediateOutputPath)</IntermediateOutputPath>
    <AppendTargetFrameworkToOutputPath>true</AppendTargetFrameworkToOutputPath>
  </PropertyGroup>
</Project>

3. Initialize SpacetimeDB module in apps/api:

cd apps/api
spacetime init --lang csharp .

4. Try to build:

spacetime build --project-path .

Current Behavior

Build appears to succeed but errors with: "Built project successfully but couldn't find the output file"

Expected Behavior

Either:

1. The build command should respect the output paths configured in Directory.Build.props, or
2. The build command could be simplified to just handle compilation without needing to locate the output files. Any commands that need the built files (like generate) could handle file location separately.

Environment

• SpacetimeDB CLI version: 1.0.0
• Project type: C#

Start with the spacetime build --project-path . entry point and reproduce the C# module under apps/api with the root Directory.Build.props. Trace how the build command locates the compiled output when custom OutputPath and AppendTargetFrameworkToOutputPath are set. Done means the command finds the output or cleanly separates compilation from later file-location needs.