GL-7: Scope ARCHITECTURE.md forbidden imports as aspirational

Existing code has legacy violations of the REST exception import rule.
Add note acknowledging these as tech debt to be migrated over time,
not blockers for new contributions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Ken Stevens

ARCHITECTURE.md

@@ -38,14 +38,16 @@ As you cross boundaries in an application, the primary domain model shifts. Each
 
 ### Forbidden Imports in Core Modules
 
-The following modules MUST NOT import from `ca.uhn.fhir.rest.server.exceptions`:
+**New code** in the following modules MUST NOT import from `ca.uhn.fhir.rest.server.exceptions`:
 
 - `cqf-fhir-cr`
 - `cqf-fhir-cql`
 - `cqf-fhir-utility`
 
 These modules are used by the CLI, by Android clients, and by other non-REST consumers. REST/HTTP concepts do not belong here.
 
+> **Note:** Existing code has legacy violations of this rule. These should be migrated over time but are not blockers for new contributions.
+
 ### Exception Handling Pattern
 
 **Core modules** throw domain-appropriate exceptions: