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/entry_test.go

@@ -65,7 +65,7 @@ func TestEntryValidation(t *testing.T) {
 		{"key_empty", &WalEntry{OpPut, VKInline, []byte{}, []byte("v")}, true},
 		{"key_too_big", &WalEntry{OpPut, VKInline, bigKey, []byte("v")}, true},
 		{"put_inline_val_too_big", &WalEntry{OpPut, VKInline, []byte("k"), bigVal}, true},
-		{"put_vlptr_val_empty", &WalEntry{OpPut, VKValueLogPointer, []byte("k"), []byte{}}, true},
+		{"put_inline_val_empty", &WalEntry{OpPut, VKInline, []byte("k"), []byte{}}, false},
 	}
 
 	for _, tc := range cases {