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.

schemas/federation.schema.yaml

@@ -0,0 +1,44 @@
+$schema: https://json-schema.org/draft/2020-12/schema
+$id: https://reuse-surface.local/schemas/federation.schema.yaml
+title: Registry Federation Manifest
+description: >
+  Schema for registry/federation/sources.yaml. Describes local and sibling
+  capability index sources to compose into a federated index.
+type: object
+additionalProperties: false
+required: [version, domain, collision_policy, sources]
+properties:
+  version:
+    type: integer
+    minimum: 1
+  domain:
+    type: string
+  collision_policy:
+    type: string
+    enum: [warn, fail]
+  sources:
+    type: array
+    minItems: 1
+    items:
+      $ref: '#/$defs/source'
+$defs:
+  source:
+    type: object
+    additionalProperties: false
+    required: [repo, index, enabled]
+    properties:
+      repo:
+        type: string
+        minLength: 1
+      index:
+        type: string
+        minLength: 1
+      enabled:
+        type: boolean
+      required:
+        type: boolean
+        default: false
+      domain:
+        type: string
+      description:
+        type: string