Expose input destinations through the public facade

docs/config.md

@@ -149,6 +149,33 @@ have index zero. Explicit empty groups and `NoSource` consume no source ID.
 Byte, source, node, and glob-entry budgets are shared across the load, while
 the existing per-source and import-depth bounds still apply.
 
+#### Input destinations
+
+`Input.Destination` places the source's application mapping beneath a path
+relative to the configuration root. Its zero or empty value means root, so
+existing inputs keep their behavior. Main inputs and underlay inputs use the
+same destination model. Destination segments are literal mapping keys: dots,
+slashes, and numeric-looking text are not split or interpreted as sequence
+indexes. Explicit index segments and paths that introduce the reserved
+`configkit` or mapping-valued `extends` structure are invalid input options and
+produce a structured `LoadError` before source acquisition.
+
+Source-root `configkit.imports` directives are processed before placement, and
+each imported application contribution inherits its input's destination,
+including transitive imports. A valid destination on `NoSource()` contributes
+nothing; an actual empty mapping materializes its destination containers. Once
+placed, contributions follow the same strict composition within a tier and
+recursive precedence behavior described above. A selected overlay and CLI
+overrides remain rooted at the application root and are not moved beneath the
+main input's destination.
+
+`SourceInfo.Destination` reports the placement for each acquired input source,
+including imports. Placement does not change source IDs, acquisition identity,
+physical source locations, or passive resource contexts. Introduced ancestor
+mappings use the contributing document's root location as a source fallback;
+authored content retains its original source coordinates. Returned source
+records are detached copies.
+
 The maintained [layered consumer example](consumers/README.md#native-filesystem-and-layered-input)
 shows a main host file, two underlay tiers, embedded defaults, a mapping source,
 and explicit resource access without a second configuration pass.