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---

modules/registry.json

@@ -62,6 +62,13 @@
       "deprecated": false
     }
   },
+  "rds": {
+    "1.0.0": {
+      "interface": "modules/l1/rds/interface.json",
+      "published_at": "2026-07-22T20:00:00Z",
+      "deprecated": false
+    }
+  },
   "static-assets": {
     "1.0.0": {
       "interface": "modules/l2/static-assets/composition.json",