Complete WP-0010: HTTP remote federation with cache

Extend federation manifest schema for url sources with auth and TTL metadata.
Fetch remote capability indexes over HTTP(S), cache under
registry/federation/cache/, and fall back to stale cache on fetch failure.
Add --refresh flag, seven federation tests, and updated federation docs.

registry/indexes/federated.yaml

@@ -6,8 +6,8 @@ domain: helix_forge
 collision_policy: warn
 sources:
 - repo: reuse-surface
-  index: registry/indexes/capabilities.yaml
   count: 12
+  index: registry/indexes/capabilities.yaml
 capabilities:
 - id: capability.activity.event-coordinate
   name: Organizational Event Coordination