Complete REUSE-WP-0005: registry federation and relation graphs

Add federation manifest and schema, federation compose and graph CLI commands,
relation cycle/reference checks, federated index and Mermaid graph artifacts,
RegistryFederation guide, and CI validation updates.

registry/federation/sources.yaml

@@ -0,0 +1,35 @@
+# Federation manifest for helix_forge capability indexes.
+# Compose with: reuse-surface federation compose
+version: 1
+domain: helix_forge
+collision_policy: warn
+
+sources:
+  - repo: reuse-surface
+    index: registry/indexes/capabilities.yaml
+    enabled: true
+    required: true
+    domain: helix_forge
+    description: Primary local capability registry
+
+  # Enable when sibling repos publish registry/indexes/capabilities.yaml
+  - repo: state-hub
+    index: ~/state-hub/registry/indexes/capabilities.yaml
+    enabled: false
+    required: false
+    domain: helix_forge
+    description: State Hub coordination capabilities
+
+  - repo: feature-control
+    index: ~/feature-control/registry/indexes/capabilities.yaml
+    enabled: false
+    required: false
+    domain: helix_forge
+    description: Feature control domain capabilities
+
+  - repo: identity-canon
+    index: ~/identity-canon/registry/indexes/capabilities.yaml
+    enabled: false
+    required: false
+    domain: helix_forge
+    description: Identity canon research capabilities