feat(P27): RDS primitive + adapter expansion (D-059)

Add modules/l1/rds/ with interface.json (engine enum for postgres, mysql,
mariadb, sqlserver, oracle), instance.json, README.md (full template with
compliance extension points). Register in registry.json. Expand the
adapter TYPE_MAP/INPUT_MAP/OUTPUT_MAP for aws:rds:instance -> aws_db_instance;
emit backup_retention_period, deletion_protection, storage_encrypted, and
skip_final_snapshot from NFRs/inputs. Add RDS to the primitives-plan matrix.
Update tests for the new registry entry count (12) + RDS adapter emission.

---ci---
project: acdl
phase: 27
milestone: v1.7
status: execute
---/ci---

.github/workflows/primitives-plan.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        primitive: [s3, vpc, ecs-cluster, ecs-service, iam-role, alb, ecr, cloudfront, waf]
+        primitive: [s3, vpc, ecs-cluster, ecs-service, iam-role, alb, ecr, cloudfront, waf, rds]
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5