No additional content is installed, so sceAppContentAddcontMount returns
SCE_APP_CONTENT_ERROR_NOT_FOUND with the mount point untouched, and
sceAppContentAddcontUnmount finds no mount to release. A missing label
or mount point is a parameter error. The library gets its own
CMakeLists for the new guest_appcontent test.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>