Compose grouped underlays in shared load lifecycle

internal/engine/records.go

@@ -6,6 +6,7 @@ type Layer string
 const (
 	LayerDefault    Layer = "default"
 	LayerBase       Layer = "base"
+	LayerUnderlay   Layer = "underlay"
 	LayerOverlay    Layer = "overlay"
 	LayerCLI        Layer = "cli"
 	LayerResolution Layer = "resolution"
@@ -101,6 +102,7 @@ const (
 	CodeImportRepeated      Code = "import.repeated"
 	CodeImportLimitExceeded Code = "import.limit_exceeded"
 	CodeImportVersion       Code = "import.version"
+	CodeUnderlayVersion     Code = "underlay.version"
 
 	CodeOverlayInvalid       Code = "overlay.invalid"
 	CodeOverlayMissing       Code = "overlay.missing"
@@ -173,12 +175,13 @@ type Location struct {
 // namespace-relative filesystem path; synthetic sources have no Path.
 // ImportedFrom identifies the declaration that included this source.
 type SourceInfo struct {
-	ID           string
-	Label        string
-	Path         string
-	Layer        Layer
-	Kind         SourceKind
-	ImportedFrom *Location
+	ID            string
+	Label         string
+	Path          string
+	Layer         Layer
+	Kind          SourceKind
+	UnderlayIndex int
+	ImportedFrom  *Location
 }
 
 // Diagnostic is a safe structured problem report. Message and source labels must