Move engine composition into the internal engine package

docs/internal/overview.md

@@ -11,13 +11,14 @@ contributor workflow and validation.
 
 | Component | Implemented responsibility | References |
 | --- | --- | --- |
-| Root `promptkit` package | Provides the supported engine facade, source, backend-registration, and injection options, public request, result, prompt-inspection, and profile-inspection values, opaque prepared-execution handles, profile construction, extension interfaces, value conversion, redacted formatting, typed capacity and generation error mapping, engine-local profile-source assembly including application fallbacks, and bounded output-repair assembly. | [Package GoDoc](../../doc.go), [prepared execution](../../prepared_execution.go), [backend API](../../backends.go), [engine assembly](../../engine.go) |
+| Root `promptkit` package | Provides the supported engine facade, source, backend-registration, and injection options, public request, result, prompt-inspection, and profile-inspection values, opaque prepared-execution handles, profile construction, extension interfaces, value conversion, redacted formatting, and typed capacity and generation error mapping. | [Package GoDoc](../../doc.go), [prepared execution](../../prepared_execution.go), [backend API](../../backends.go), [engine facade](../../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` | Constructs each engine's immutable registry from maintained definitions and consumer additions, validates and defensively copies definitions through the shared JSON-value package, and consumes the LLM-owned OpenAI-compatible reserved request-field rule. | [Backend registry](../../internal/backend/registry.go) |
 | `internal/catalog` | Strictly validates imported immutable maintained backend and profile catalog assets before engine assembly uses them. | [Catalog adapter](../../internal/catalog/catalog.go), [internal sources](sources.md#profiles-and-built-ins) |
 | `internal/capacity` | Owns engine-local bounded execution admission and FIFO model-generation permits for limited backend IDs, including cancellation-safe waiter removal and client wrapping. | [Internal capacity management](capacity.md) |
 | `internal/domain` | Defines internal framework values for requests, artifacts, prompt definitions, profiles, execution targets, rendering, generation, and validation, and owns source-neutral invariants for shared execution settings, OpenAI-compatible base endpoints, session identifiers, and output contracts. Source parsing, required fields, other source-specific normalization, defaulting, and boundary-specific error classification remain with their callers. | [Domain declarations](../../internal/domain/domain.go), [endpoint invariant](../../internal/domain/endpoint.go) |
+| `internal/engine` | Assembles catalogs, repositories, registry, capacity, collaborators, repairer, and the one use-case runner for each engine instance. | [Engine assembly](../../internal/engine/engine.go), [internal runner](runner.md) |
 | `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) |
 | `internal/jsonvalue` | Validates and deeply copies bounded JSON-compatible extra-parameter and prepared-schema trees while preserving supported concrete value types and rejecting cycles or excessive depth and work. | [JSON values](../../internal/jsonvalue/jsonvalue.go) |

docs/internal/runner.md

@@ -9,8 +9,8 @@ artifact, rendering, and validation behavior, while the
 [model-client document](llm.md) owns generation behavior and failure
 categories.
 
-The runner remains under `internal/usecase` and is assembled by the root
-Promptkit engine. Its concrete type is not part of the public API.
+The runner remains under `internal/usecase` and is assembled by
+`internal/engine`. Its concrete type is not part of the public API.
 The [framework format reference](../formats.md) owns prompt, profile, schema,
 and override semantics consumed by the runner.
 
@@ -18,12 +18,12 @@ and override semantics consumed by the runner.
 
 `Runner` coordinates narrow internal interfaces for prompt definitions,
 profiles, backend resolution, artifacts, rendering, model generation, and
-validation. The root engine supplies one immutable registry containing the
+validation. The internal engine supplies one immutable registry containing the
 built-in backend and validated consumer additions, one engine-local run
 admitter, and a model client wrapped by the same capacity manager. Validation
 plans and provider-facing schema metadata come from the validator's preparation
 interface.
-The root engine supplies one default output repairer through the explicit
+The internal engine supplies one default output repairer through the explicit
 runner constructor, using the same capacity-wrapped client as initial
 generation. The no-repair runner constructor remains available for focused
 internal callers and tests.

docs/internal/sources.md

@@ -6,7 +6,7 @@ This document describes Promptkit's implemented internal source, artifact,
 rendering, and output-validation behavior. The
 [architecture policy](../policy/architecture.md) owns the library boundary and
 dependency rules. None of these internal packages is a supported consumer API,
-and the root engine assembles them behind its public source options and values.
+and the internal engine assembles them behind the public facade's source options and values.
 The [framework format reference](../formats.md) owns the exact file fields,
 validation modes, built-in catalog, and source precedence.
 
@@ -80,7 +80,7 @@ backend registry membership because the available registry belongs to the
 assembled engine; the runner checks membership during preparation and exact
 profile inspection.
 
-The root engine assembles one raw composite catalog in precedence order:
+The internal engine assembles one raw composite catalog in precedence order:
 in-memory profiles, one ordinary configured source, an application fallback
 source, then the maintained external catalog. An explicit file or `fs.FS` profile
 source replaces `Config.ProfileDir` within the ordinary configured-source