Complete input destination implementation audit

docs/consumers/README.md

@@ -111,6 +111,11 @@ lifetime and precedence rules.
 
 ## Input destinations and mounted collections
 
+Main and underlay inputs use the same destination field; imported application
+contributions inherit the declaring input's destination. A destination is an
+application mapping path, independent of filesystem context. The configuration
+contract owns the complete behavior and validation rules.
+
 The small [`Example_inputDestination`](../../examples/consumers/destinations_test.go)
 mounts one mapping source beneath literal collection/prompt keys and loads a
 second input without a destination to show that root placement remains the