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.

workplans/REUSE-WP-0010-network-federation.md

@@ -4,7 +4,7 @@ type: workplan
 title: "Network federation for remote indexes"
 domain: helix_forge
 repo: reuse-surface
-status: backlog
+status: finished
 owner: codex
 topic_slug: helix-forge
 created: "2026-06-15"
@@ -21,7 +21,7 @@ fetch capability indexes from HTTP URLs or git raw endpoints.
 
 ```task
 id: REUSE-WP-0010-T01
-status: todo
+status: done
 priority: medium
 state_hub_task_id: "6f181057-e0f7-4879-9eb9-928a527a01ee"
 ```
@@ -33,7 +33,7 @@ sources alongside `index` file paths, with optional auth and TTL metadata.
 
 ```task
 id: REUSE-WP-0010-T02
-status: todo
+status: done
 priority: medium
 state_hub_task_id: "a2fac7d5-9383-4a42-bd23-3e8dbc7d550b"
 ```
@@ -45,7 +45,7 @@ Add HTTP fetch to `federation compose` with local cache under
 
 ```task
 id: REUSE-WP-0010-T03
-status: todo
+status: done
 priority: low
 state_hub_task_id: "73996193-ecae-4fb4-84f7-fe84a5cd8898"
 ```