feat(federation): register config-atlas and reserve surface.* namespace

Add config-atlas as a federation source (capability index) and to the local
repo roster. Document surface.* as a distinct id namespace owned by
config-atlas (typed sibling of capability.*, not federated here) under a new
"Id namespace ownership" section. Raw URL currently 303 (required:false), same
publish block as state-hub/feature-control. Recompose federated.yaml.

Supports config-atlas ATLAS-WP-0002-T05.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

registry/federation/sources.yaml

@@ -58,6 +58,18 @@ sources:
   domain: helix_forge
   cache_ttl_seconds: 86400
   auth_header: Authorization
+- repo: config-atlas
+  url: https://gitea.coulomb.social/coulomb/config-atlas/raw/main/registry/indexes/capabilities.yaml
+  enabled: true
+  required: false
+  domain: helix_forge
+  description: >-
+    Configuration Surface Atlas. Owns the `surface.*` id namespace (configuration
+    surfaces in registry/indexes/surfaces.yaml) as a typed sibling of `capability.*`;
+    only its capability index is federated here. Raw URL returns 303 until publish
+    is resolved (same block as state-hub/feature-control).
+  cache_ttl_seconds: 86400
+  auth_header: Authorization
 - repo: coordination-engine
   url: https://gitea.coulomb.social/coulomb/coordination-engine/raw/main/registry/indexes/capabilities.yaml
   enabled: true