fix: add write-stopped test and remove VKValueLogPointer from test logic

- Add TestWriteStoppedAfterIOError verifying ErrCommitUnknown → write-stopped
  transition after I/O failure (DoD requirement)
- Replace VKValueLogPointer with VKInline in segment_writer_test.go large
  batch test (9 × 4KB entries, still tests multi-fragment)
- Replace put_vlptr_val_empty with put_inline_val_empty in entry_test.go
- VKValueLogPointer remains only in constants_test.go (enum value check)

wal/writer_test.go

@@ -1,11 +1,13 @@
 package wal
 
 import (
+	"errors"
 	"fmt"
 	"sync"
 	"testing"
 
 	"github.com/dailz/go-kv/config"
+	"github.com/dailz/go-kv/errkit"
 )
 
 func newTestWalWriter(t *testing.T) *WalWriter {
@@ -118,3 +120,47 @@ func TestWalWriterDelete(t *testing.T) {
 		t.Fatalf("Get(k) found deleted key with value %q", got.Value)
 	}
 }
+
+// TestWriteStoppedAfterIOError verifies: I/O failure during AppendBatch →
+// ErrCommitUnknown returned, write-stopped state entered, subsequent writes
+// return ErrWriteStopped.
+func TestWriteStoppedAfterIOError(t *testing.T) {
+	cfg := config.Defaults()
+	ww, err := NewWalWriter(&cfg, t.TempDir(), 0, 0)
+	if err != nil {
+		t.Fatalf("NewWalWriter: %v", err)
+	}
+
+	if err := ww.Put([]byte("ok-key"), []byte("ok-val")); err != nil {
+		t.Fatalf("initial Put: %v", err)
+	}
+
+	// Close the fd to force AppendBatch → stopWithError(ErrCommitUnknown).
+	if err := ww.segManager.active.fd.Close(); err != nil {
+		t.Fatalf("close fd: %v", err)
+	}
+
+	err = ww.Put([]byte("fail-key"), []byte("fail-val"))
+	if err == nil {
+		t.Fatal("expected error from Put after I/O failure, got nil")
+	}
+	if !errors.Is(err, errkit.ErrCommitUnknown) {
+		t.Errorf("Put error = %v, want ErrCommitUnknown", err)
+	}
+
+	if !ww.IsWriteStopped() {
+		t.Fatal("IsWriteStopped() = false, want true after I/O error")
+	}
+
+	err = ww.Put([]byte("after-key"), []byte("after-val"))
+	if !errors.Is(err, errkit.ErrWriteStopped) {
+		t.Errorf("Put after write-stopped error = %v, want ErrWriteStopped", err)
+	}
+
+	err = ww.Delete([]byte("after-key"))
+	if !errors.Is(err, errkit.ErrWriteStopped) {
+		t.Errorf("Delete after write-stopped error = %v, want ErrWriteStopped", err)
+	}
+
+	_ = ww.Close()
+}