Add immutable backend registry foundation

docs/internal/overview.md

@@ -14,6 +14,7 @@ contributor workflow and validation.
 | Root `promptkit` package | Provides the supported engine facade, source and injection options, public request and result values, built-in profile construction, extension interfaces, value conversion, redacted formatting, and public error mapping. | [Package GoDoc](../../doc.go), [engine assembly](../../engine.go) |
 | `examples/go-library/prepare` | Demonstrates an offline downstream consumer using a prompt file, in-memory profile, inline input, and `Prepare`. It is not a public library package. | [Example program](../../examples/go-library/prepare/main.go) |
 | `examples/go-library/run` | Demonstrates an offline downstream consumer using a prompt file, in-memory profile, inline input, an injected deterministic model client, and `Run`. It is not a public library package. | [Example program](../../examples/go-library/run/main.go) |
+| `internal/backend` | Validates and defensively copies immutable OpenAI-compatible backend definitions, supplies the built-in OpenRouter definition, and owns the shared reserved request-field rule. | [Backend registry](../../internal/backend/registry.go) |
 | `internal/domain` | Defines internal framework values for requests, artifacts, prompt definitions, profiles, execution targets, rendering, generation, and validation. | [Domain declarations](../../internal/domain/domain.go) |
 | `internal/defaults` | Defines application-neutral framework constants and constructs the default execution target. It contains no CLI, server, or inbound HTTP limits. | [Framework defaults](../../internal/defaults/defaults.go) |
 | `internal/filecatalog` | Provides deterministic YAML discovery and path helpers for operating-system filesystems and `fs.FS` sources. | [File catalog](../../internal/filecatalog/catalog.go) |

docs/policy/architecture.md

@@ -21,6 +21,8 @@ The implemented internal components consist of:
 
 - `internal/domain`, which owns framework data values shared by later internal
   components;
+- `internal/backend`, which owns validated immutable OpenAI-compatible backend
+  definitions and the built-in OpenRouter definition;
 - `internal/defaults`, which owns application-neutral framework defaults and
   constructs the default execution target;
 - `internal/filecatalog`, which discovers YAML files and provides source-path
@@ -72,9 +74,11 @@ downstream consumers, including Scriptorium
         narrow injected abstractions
 ```
 
-The facade coordinates internal components and adapts the supported public
-extension interfaces to narrow internal abstractions. Internal components must
-not depend on consumers or on Scriptorium.
+The backend registry depends on the domain model, and the model client reuses
+its OpenAI-compatible reserved request-field rule. The facade coordinates
+internal components and adapts the supported public extension interfaces to
+narrow internal abstractions. Internal components must not depend on consumers
+or on Scriptorium.
 
 ## Repository And Consumer Boundary
 

docs/roadmap/implementation.md

@@ -245,6 +245,8 @@ where the existing error boundary permits it.
 
 ## Stage 1 — Registry Foundation
 
+**Status:** Complete.
+
 ### Goal
 
 Introduce the internal domain and immutable registry without changing the