Document and demonstrate native and layered configuration loading

docs/config.md

@@ -148,6 +148,10 @@ 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.
 
+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.
+
 ### Source resource contexts
 
 `Result.SourceContext(id)` and callback-scoped `Resolution.SourceContext(id)`
@@ -165,6 +169,12 @@ contextless synthetic inputs have no resource context. To open a relative
 resource, join `BaseDir` with the application resource name using slash-path
 operations and open that path through `FS`.
 
+For a resource-valued field, query `result.Metadata().Lookup(fieldPath)`, inspect
+its origin locations, choose a source ID only when the application's policy can
+identify the intended contributor, then call `result.SourceContext(id)`. The
+context belongs to that source even when the field survived from a lower-priority
+underlay. A composite origin has no automatically selected resource base.
+
 Native contexts borrow the caller's filesystem within the configured root.
 Host contexts retain a canonical root path, not an open root handle; each
 resource `Open` checks containment and opens a fresh handle that the returned