Centralize parser diagnostic translation

docs/internal/overview.md

@@ -10,6 +10,7 @@ module and package documentation boundary. Its implemented public primitives are
 - a parser-independent ordered tree in `internal/document`;
 - the sole upstream YAML integration in `internal/yaml`, which translates validated
   input into that owned tree and exposes no parser types outside its boundary;
+  `parse.go` owns shared translation into detached, ordered public diagnostics;
 - a private strict composer in `compose.go` that joins only disjoint document-tree
   definitions, retains ordered claims and composite mapping origins, reports every
   conflicting location, and never aliases an input tree;