Centralize execution setting and session validation

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