Expose sequence placement controls in the public API

layered.go

@@ -12,8 +12,12 @@ type Input struct {
 	// path or append a relative destination declared in YAML. Its zero value
 	// keeps root placement.
 	Destination Path
-	Imports     bool
-	Filesystem  *FilesystemContext
+	// PlacementMode defaults to mapping placement. Sequence-item placement wraps
+	// the source mapping as one record; unknown values and an empty destination
+	// fail option validation before acquisition.
+	PlacementMode PlacementMode
+	Imports       bool
+	Filesystem    *FilesystemContext
 }
 
 // Underlay contains inputs that must compose strictly within one tier. Tiers
@@ -55,10 +59,11 @@ func LoadLayered[T any](main Input, supplied ...LayeredOptions[T]) (*Result[T],
 }
 
 func toEngineInput(input Input) engine.Input {
-	return engine.Input{
+	result := engine.Input{
 		Source: input.Source.toEngine(), Imports: input.Imports,
 		Filesystem: toEngineFilesystemContext(input.Filesystem), Destination: toEnginePath(input.Destination),
 	}
+	return engine.InputWithPlacement(result, uint8(input.PlacementMode))
 }
 
 func toEngineLayeredOptions[T any](options LayeredOptions[T]) engine.LayeredOptions[T] {