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/debounce.go

@@ -0,0 +1,42 @@
+package graph
+
+import (
+	"sync"
+	"time"
+)
+
+// Debouncer coalesces rapid calls with the same key, executing only the last
+// one after a configurable delay. This prevents redundant work when multiple
+// updates arrive in quick succession (e.g., rapid schema publishing).
+type Debouncer struct {
+	mu     sync.Mutex
+	delay  time.Duration
+	timers map[string]*time.Timer
+}
+
+// NewDebouncer creates a Debouncer with the given delay window.
+func NewDebouncer(delay time.Duration) *Debouncer {
+	return &Debouncer{
+		delay:  delay,
+		timers: make(map[string]*time.Timer),
+	}
+}
+
+// Debounce resets the timer for key. When the timer fires (after delay with no
+// new calls for the same key), fn is executed in a new goroutine.
+func (d *Debouncer) Debounce(key string, fn func()) {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+
+	if t, ok := d.timers[key]; ok {
+		t.Stop()
+	}
+
+	d.timers[key] = time.AfterFunc(d.delay, func() {
+		d.mu.Lock()
+		delete(d.timers, key)
+		d.mu.Unlock()
+
+		fn()
+	})
+}