Harden backend contracts and documentation

docs/consumers/pkg-promptkit.md

@@ -33,8 +33,8 @@ engine, err := promptkit.NewEngine(promptkit.Config{
 })
 ```
 
-Options support single-file or `fs.FS` sources, in-memory profiles, and
-injected artifact or model clients. Consult the
+Options support single-file or `fs.FS` sources, in-memory profiles,
+engine-scoped backends, and injected artifact or model clients. Consult the
 [constructor and option GoDoc](../../engine.go) for composition, precedence,
 validation, and default transport behavior. Source discovery, format
 validation, and profile precedence are defined by the
@@ -123,6 +123,13 @@ Registrations belong to one engine and custom IDs cannot replace built-ins.
 The [`Backend` and `WithBackend` GoDoc](../../backends.go) defines validation,
 copying, uniqueness, and request-default behavior.
 
+Both file-backed and in-memory profiles select a registration through
+`backend` or `Profile.BackendID`. Profile and request endpoint overrides retain
+that routing identity. `PreparedRun.SelectedBackendID`,
+`RunResult.SelectedBackendID`, and the effective `ExecutionTarget.BackendID`
+expose it to consumers and injected model clients. Endpoint-only profiles
+remain supported and expose an empty backend ID.
+
 ## Credentials
 
 File-backed profiles name an environment variable; in-memory profiles can
@@ -163,7 +170,8 @@ distinguish invalid construction, invalid requests, absent sources,
 source-loading failures, collaborator failures, and operational validation
 failures. Specific request conditions may also match the broader
 `ErrInvalidRequest`, and injected collaborator identities are preserved where
-documented.
+documented. Invalid or duplicate backend registrations match
+`ErrInvalidConfig`; selecting an unknown backend matches `ErrProfileLoad`.
 
 ## Application Boundary