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---

tests/test_pipeline_contract.py

@@ -437,7 +437,7 @@ class TestPlatformWorkflows:
         wf = _load_workflow(".github/workflows/primitives-plan.yml")
         job = wf["jobs"]["primitive-plan"]
         matrix = job["strategy"]["matrix"]
-        expected = ["s3", "vpc", "ecs-cluster", "ecs-service", "iam-role", "alb", "ecr", "cloudfront", "waf"]
+        expected = ["s3", "vpc", "ecs-cluster", "ecs-service", "iam-role", "alb", "ecr", "cloudfront", "waf", "rds"]
         assert sorted(matrix["primitive"]) == sorted(expected)
 
     def test_primitives_plan_runs_run_primitive_plan(self):