Ensure csproj attestation

Re-runs dotnet restore against each .csproj whose MSBuildProject marker is stale (set by any csproj-mutating recipe in the run) and refreshes the marker from the resulting project.assets.json. Use this at the end of a composite recipe whose csproj-mutating sub-recipes have RegenerateMarker = false, so reattestation happens once on the final consistent state instead of after every edit. Unmodified .csproj files incur no dotnet restore cost.