fix: prevent OOM on rapid schema publishing

Add concurrency-limited CosmoGenerator (semaphore limit=1, 60s timeout)
to prevent unbounded concurrent wgc process spawning. Add debouncer
(500ms) to coalesce rapid schema updates per org+ref. Fix double
subgraph fetch in Supergraph resolver and goroutine leak in
SchemaUpdates subscription.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

graph/resolver.go

@@ -24,11 +24,13 @@ type Publisher interface {
 }
 
 type Resolver struct {
-	EventStore eventsourced.EventStore
-	Publisher  Publisher
-	Logger     *slog.Logger
-	Cache      *cache.Cache
-	PubSub     *PubSub
+	EventStore     eventsourced.EventStore
+	Publisher      Publisher
+	Logger         *slog.Logger
+	Cache          *cache.Cache
+	PubSub         *PubSub
+	CosmoGenerator *CosmoGenerator
+	Debouncer      *Debouncer
 }
 
 func (r *Resolver) apiKeyCanAccessRef(ctx context.Context, ref string, publish bool) (string, error) {