vfs/cache: Fix promoteToFast race with WriteCloser Close

promoteToFast read FileInfo.Size from Stat after the VFS lock was
released, while WriteCloser.Close updated Size on the same live
object. Concurrent Create/Open on overlapping keys tripped the race
detector.

Stat now returns a FileInfo snapshot, and promotion uses the
ReadAll length for the fast-tier Create size. Promotion stays
best-effort.

#21

vfs/memory/memory.go

@@ -289,7 +289,8 @@ func (m *MemoryFS) Delete(key string) error {
 	return nil
 }
 
-// Stat returns file information
+// Stat returns a snapshot of file information. The returned *FileInfo is not
+// the live cache entry; Close may update Size on the in-map object under m.mu.
 func (m *MemoryFS) Stat(key string) (*types.FileInfo, error) {
 	if key == "" {
 		return nil, vfserror.ErrInvalidKey
@@ -310,7 +311,7 @@ func (m *MemoryFS) Stat(key string) (*types.FileInfo, error) {
 	defer m.mu.RUnlock()
 
 	if fi, ok := m.info[key]; ok {
-		return fi, nil
+		return fi.Clone(), nil
 	}
 
 	return nil, vfserror.ErrNotFound

vfs/memory/memory_test.go

@@ -346,6 +346,45 @@ func TestMemoryFS_ConcurrentCloseAndEvict_RaceFree(t *testing.T) {
 	_ = m.LRU.Len()
 }
 
+func TestMemoryFS_StatReturnsSnapshot(t *testing.T) {
+	t.Parallel()
+	m, err := New(1024)
+	if err != nil {
+		t.Fatal(err)
+	}
+	w, err := m.Create("k", 10)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := w.Write([]byte("hello")); err != nil {
+		t.Fatal(err)
+	}
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	fi, err := m.Stat("k")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fi.Size != 5 {
+		t.Fatalf("size %d want 5", fi.Size)
+	}
+	fi.Size = 999
+	fi.AccessCount = 0
+
+	fi2, err := m.Stat("k")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fi2.Size != 5 {
+		t.Errorf("Stat returned live FileInfo; store size became %d", fi2.Size)
+	}
+	if fi2.AccessCount == 0 {
+		t.Error("Stat returned live FileInfo; AccessCount mutation leaked")
+	}
+}
+
 func TestMemoryFS_EvictVariantsAndErrors(t *testing.T) {
 	t.Parallel()
 	m, err := New(800)