Comment on multiline map key is moved to value

### Description

When formatting YAML that uses a **multiline block scalar as a map key**, comments attached to the key are not preserved.

---

### Reproduction

**Input:**

```yaml
? | # comment
multi
line
key
: value
```

**After `yamlfmt`:**

```yaml
? |
multi
line
key
: value # comment
```

The comment originally associated with the **multiline key (`? |`)** is moved to the value line.

---

### Expected behavior

The comment should remain attached to the multiline key, or the formatter should report that this construct is unsupported instead of silently moving the comment.