Implement strict document composition

docs/internal/overview.md

@@ -10,6 +10,9 @@ 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;
+- 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;
 - a per-operation private reflection descriptor graph in `schema.go` for supported
   types, YAML field binding, inlining, secret tags, recursion, and semantic paths;
 - descriptor-driven canonical projection, detached typed cloning, cycle/limit