Allow absent inputs to use import contexts

internal/engine/load.go

@@ -21,8 +21,9 @@ type Definition[T any] struct {
 }
 
 // Options contains the application definition and per-load input selections.
-// Imports enables explicit import declarations; Overlay selects one fixed overlay;
-// Overrides contains only values explicitly supplied by an application CLI.
+// Imports enables traversal of declarations in a selected main document; an
+// absent source has no main declarations to traverse. Overlay selects one fixed
+// overlay. Overrides contain only values explicitly supplied by an application CLI.
 type Options[T any] struct {
 	Definition              Definition[T]
 	Imports                 bool
@@ -39,7 +40,9 @@ type Options[T any] struct {
 	importDefaultsValidated bool
 }
 
-// FilesystemContext supplies filesystem roots for imports and overlays.
+// FilesystemContext supplies filesystem roots for imports and overlays. For an
+// absent host input, Root is required and BaseDir may be omitted to use Root as
+// the import base. Native contexts still require both BaseDir and Root.
 type FilesystemContext struct {
 	BaseDir string
 	Root    string