bird: leading-edge reload + 500ms cooldown (was trailing 500ms debounce)

A single Ready/NotReady transition no longer pays a 500ms reload wait —
the first call to scheduleReload fires birdc immediately; further calls
within 500ms are coalesced into one tail reload at the cooldown's end.

Burst behavior is the same as before: under heavy churn (deploy rolling
all replicas at once), at most one reload per 500ms.

Steady-state latency from pod Ready transition to crt001 BGP withdraw:
  - probe period (set in pod spec, 1s minimum)
  - ~ms informer + reconcile + birdc + BGP UPDATE
The 500ms hardcoded delay is gone.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

pkg/agent/bird.go

@@ -24,11 +24,18 @@ type BirdManager struct {
 	BirdctlPath string // "birdc" — overridable for tests
 	Logger      *slog.Logger
 
-	mu       sync.Mutex
-	last     string // last rendered output (de-dup)
-	debounce *time.Timer
+	mu          sync.Mutex
+	last        string // last rendered output (de-dup)
+	cooldown    *time.Timer
+	cooldownEnd time.Time // window during which further reloads are coalesced
+	pending     bool      // a render landed during cooldown; reload at window end
 }
 
+// reloadCooldown is the minimum spacing between two birdc reloads. The
+// first change fires immediately (no leading-edge delay); follow-up
+// changes within this window are coalesced into a single tail reload.
+const reloadCooldown = 500 * time.Millisecond
+
 // Render writes the config from a NodeConfig + anycast set. Idempotent —
 // if the rendered content matches what we last wrote, no birdc reload.
 func (b *BirdManager) Render(nc *flockv1alpha1.NodeConfig, anycast6, anycast4 []string, routerID string) error {
@@ -88,12 +95,32 @@ func (b *BirdManager) Render(nc *flockv1alpha1.NodeConfig, anycast6, anycast4 []
 	return nil
 }
 
-// scheduleReload coalesces birdc reload calls into ~500ms windows.
+// scheduleReload uses leading-edge + cooldown semantics: the first call
+// reloads immediately; subsequent calls within reloadCooldown coalesce
+// into a single deferred reload at the cooldown's end. Caller holds b.mu.
 func (b *BirdManager) scheduleReload() {
-	if b.debounce != nil {
-		b.debounce.Stop()
+	now := time.Now()
+	if now.After(b.cooldownEnd) {
+		// Outside any active cooldown — fire now (leading edge).
+		b.cooldownEnd = now.Add(reloadCooldown)
+		b.pending = false
+		go b.reload()
+		return
 	}
-	b.debounce = time.AfterFunc(500*time.Millisecond, b.reload)
+	// Inside cooldown — coalesce. If no tail timer is set, schedule one
+	// at the cooldown end; if already set, just leave it.
+	if b.pending {
+		return
+	}
+	b.pending = true
+	delay := b.cooldownEnd.Sub(now)
+	b.cooldown = time.AfterFunc(delay, func() {
+		b.mu.Lock()
+		b.pending = false
+		b.cooldownEnd = time.Now().Add(reloadCooldown)
+		b.mu.Unlock()
+		b.reload()
+	})
 }
 
 func (b *BirdManager) reload() {