feat: allow provider-delegated reefs with no rail

Object-storage reefs omit primary_rail and hosted_rails. Scaleway
operates S3; reef-storage only names the boundary. Finish
RMASTER-WP-0022.

tools/testdata/family-declarations/good/reef-storage-example/declarations/reef.yaml

@@ -0,0 +1,13 @@
+kind: substrate-reef
+reef_id: reef-storage-example
+repo: reef-storage-example
+ownership_repo: railiance-platform
+substrate_kind: object-storage
+lifecycle_state: planned
+criticality: high
+current_members: []
+source_documents:
+  - repo: railiance-master
+    path: docs/reef-substrate-model.md
+compatibility_notes:
+  - Provider-delegated S3 fixture. No rail. The provider operates the capability.

tools/validate-family-declarations.py

@@ -190,7 +190,7 @@ def check_reef(loaded: Loaded, report: Report) -> None:
         report.add(loaded.rel, "reef_id", f"{doc['reef_id']!r} must equal repo {doc['repo']!r}")
     primary = doc.get("primary_rail")
     hosted = as_list(doc.get("hosted_rails"))
-    if primary and hosted and primary not in hosted:
+    if primary and primary not in hosted:
         report.add(
             loaded.rel,
             "primary_rail",