"Hand-vendored library without an .sbproj is silently skipped"

A hand-vendored library placed under Libraries/ compiles with no errors in an offline dotnet build, but the game code fails the editor compile with a wall of CS0246 ("type or namespace not found") on every type the library defines. The library's own code never appears in compile_status at all, as though it does not exist.

Library discovery runs at project load (LibrarySystem.InitializeFromProject in the engine's Sandbox.Tools). Each subfolder of Libraries/ is admitted only if it contains exactly one *.sbproj file. Zero files (the hand-vendored case) or two or more files both cause the folder to be silently skipped, with no warning and no log line.

A skipped folder never becomes a library project, so the game compiler receives no reference to it. The CS0246 errors are not about missing code; the library's code is present on disk but was never offered to the compiler.

Library Manager installs get their .sbproj from the package pipeline, which is why only hand-vendored or manually copied libraries hit this.

Add a minimal library-type project file to each vendored folder, matching the engine's library.minimal template:

{
  "Title": "My Library",
  "Type": "library",
  "Org": "my-org",
  "Ident": "my-library",
  "Schema": 1
}

Two sharp edges:

1. Discovery runs only at project load. Adding the .sbproj under a live editor does nothing until the project is closed and reopened. A hot recompile does not rescan Libraries/.
2. Offline dotnet build cannot catch a missing .sbproj. It compiles the library source directly via the csproj and does not simulate the editor's discovery logic. A green offline build does not prove the editor will find the library.

Once discovered, the game auto-references every loaded library with a code path. No PackageReferences entry is needed for local libraries.

The .sbproj file signals to the engine's library system that the folder is a valid library project. With exactly one project file present, AddFromFolder registers it as a library, compiles its code, and exposes its types to the game project. Without the file, the folder is structurally invisible to the discovery scan.