vfs/disk: Harden pending-window test against empty-dir race

Empty-dir disk init closes initDone almost immediately, so
TestDiskTierSignalMixedPendingReady can observe DiskTierReady=0 then
a heartbeat that already saw ready. Register a per-root init hold so
the test can keep the pending window open without slowing production.

Link: #36

steamcache/steamcache_test.go

@@ -14,6 +14,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"s1d3sw1ped/steamcache2/steamcache/metrics"
+	"s1d3sw1ped/steamcache2/vfs/disk"
 	"s1d3sw1ped/steamcache2/vfs/eviction"
 	"s1d3sw1ped/steamcache2/vfs/memory"
 	"s1d3sw1ped/steamcache2/vfs/vfserror"
@@ -1149,19 +1150,31 @@ func TestDiskTierSignalMemoryOnly(t *testing.T) {
 // TestDiskTierSignalMixedPendingReady covers mixed mode: DiskTierReady=0 (header
 // pending) while the disk attach is in the Size barrier, then DiskTierReady=1
 // (header ready) after the barrier opens and the attach goroutine sets SetSlow.
+// An init hold keeps the empty-dir attach from finishing between the pending
+// metric check and the heartbeat, which otherwise races under CI load.
 func TestDiskTierSignalMixedPendingReady(t *testing.T) {
 	td := t.TempDir()
 	diskPath := filepath.Join(td, "disk")
 	if err := os.MkdirAll(diskPath, 0755); err != nil {
 		t.Fatal(err)
 	}
+	hold := make(chan struct{})
+	var holdOnce sync.Once
+	closeHold := func() { holdOnce.Do(func() { close(hold) }) }
+	disk.RegisterInitHold(diskPath, hold)
+	t.Cleanup(func() {
+		closeHold()
+		disk.ClearInitHold(diskPath)
+	})
+
 	sc, err := New("127.0.0.1:0", "1MB", "10MB", diskPath, "", "lru", "lru", 10, 1, "0", nil, "")
 	if err != nil {
 		t.Fatalf("New mixed: %v", err)
 	}
 	t.Cleanup(func() { sc.Shutdown() })
+	t.Cleanup(closeHold) // before Shutdown: attach is blocked in Size() until the hold closes
 
-	// Immediately in the pending window
+	// Pending window is held open until closeHold; attach cannot finish.
 	if got := sc.GetMetrics().DiskTierReady; got != 0 {
 		t.Errorf("immediate DiskTierReady=%d, want 0 (pending)", got)
 	}
@@ -1172,7 +1185,7 @@ func TestDiskTierSignalMixedPendingReady(t *testing.T) {
 		t.Errorf("heartbeat header=%q, want pending", got)
 	}
 
-	// Wait the barrier, then retry until the attach goroutine flips the flag
+	closeHold()
 	_ = sc.disk.Size()
 	deadline := time.Now().Add(2 * time.Second)
 	for time.Now().Before(deadline) {