test: add waitFor helper for event-driven testing

- Add test_wait_helper.go with waitFor() function for polling-based test assertions
- Used by daemon watcher platform tests for non-blocking event verification
- Sync beads.jsonl

Amp-Thread-ID: https://ampcode.com/threads/T-80e427aa-40e0-48a6-82e0-e29a93edd444
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/test_wait_helper.go

@@ -0,0 +1,20 @@
+package main
+
+import (
+	"testing"
+	"time"
+)
+
+// waitFor repeatedly evaluates pred until it returns true or timeout expires.
+// Use this instead of time.Sleep for event-driven testing.
+func waitFor(t *testing.T, timeout, poll time.Duration, pred func() bool) {
+	t.Helper()
+	deadline := time.Now().Add(timeout)
+	for time.Now().Before(deadline) {
+		if pred() {
+			return
+		}
+		time.Sleep(poll)
+	}
+	t.Fatalf("condition not met within %v", timeout)
+}