Centralize execution setting and session validation

docs/internal/llm.md

@@ -29,7 +29,7 @@ Construction validates the configured base URL and clones any supplied
 `http.Client` so Promptkit can apply its timeout default without mutating the
 caller's client. Generation then:
 
-1. validates request-level timeout and endpoint requirements;
+1. validates shared execution-setting invariants and endpoint requirements;
 2. maps the internal request into the OpenAI-compatible chat payload;
 3. validates and merges extra parameters;
 4. resolves authentication;

docs/internal/overview.md

@@ -16,7 +16,7 @@ contributor workflow and validation.
 | `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 the built-in OpenRouter definition 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/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. | [Domain declarations](../../internal/domain/domain.go) |
+| `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 and session identifiers. Source parsing, required fields, source-specific normalization, and boundary-specific error classification remain with their callers. | [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) |
 | `internal/jsonvalue` | Validates and deeply copies JSON-compatible extra-parameter and prepared-schema trees while preserving supported concrete value types. | [JSON values](../../internal/jsonvalue/jsonvalue.go) |

docs/policy/architecture.md

@@ -19,8 +19,8 @@ results, public values, extension interfaces, profiles, and error sentinels.
 
 The implemented internal components consist of:
 
-- `internal/domain`, which owns framework data values shared by later internal
-  components;
+- `internal/domain`, which owns framework data values and source-neutral
+  invariants shared by later internal components;
 - `internal/backend`, which owns validated immutable OpenAI-compatible backend
   definitions and the built-in OpenRouter definition;
 - `internal/capacity`, which owns engine-local bounded run admission and
@@ -92,6 +92,12 @@ coordinates internal components and adapts the supported public extension
 interfaces to narrow internal abstractions. Internal components must not depend
 on consumers or on Scriptorium.
 
+`internal/domain` owns source-neutral invariants for values shared across
+multiple input and execution boundaries, including execution-setting bounds
+and session identifiers. Callers retain source parsing, required-field rules,
+source-specific normalization, error classification, and other policy specific
+to their own boundary.
+
 ## Repository And Consumer Boundary
 
 Scriptorium is a downstream application that consumes Promptkit through