* fix(sub): coalesce external subscription refreshes
Limit concurrent cache misses to one upstream request per URL and evict the oldest entries once the cache reaches its bounded capacity.
* fix(sub): preserve shared stale refresh results
Release every in-flight waiter on panic or error, carry the leader outcome to waiters, and strengthen cache capacity and stale fallback coverage.
---------
Co-authored-by: PathGao <gaoyanbo@gaoyanbodeMacBook-Air.local>