Legacy `Node`/`Link` classes not exported from `__init__.py`

The legacy `Node` and `Link` classes in `dag/dag.py` are not exported from `dag/__init__.py`, but `utils.py` references them via `node_to_link()`. This creates an inconsistency where internal code uses classes that aren't part of the public API.

### Problem

In `dag/__init__.py`:

```python
from .block import Block
from .codec import BlockCodec, BlockDecoder, BlockEncoder, ...
from .ipld_model import CID, IPLDNode, Kind, ...
from .multicodec_codes import ...

# ← Node and Link are NOT exported
```

But in `dag/utils.py`:

```python
def node_to_link(node: Any) -> Any:
from .dag import Link, Node # ← Imports from internal module

if not isinstance(node, Node):
raise TypeError("node should be an instance of type Node")

return Link("", node.size, node.multihash)
```

Users can't use `node_to_link()` without also importing from the internal `dag.dag` module.

### Proposed Solution

Either:

**Option A**: Export `Node` and `Link` from `__init__.py`:

```python
from .dag import Node, Link

__all__ = [
# ... existing exports ...
"Node",
"Link",
]
```

**Option B**: Deprecate `node_to_link()` and remove the legacy classes:

```python
import warnings

def node_to_link(node: Any) -> Any:
warnings.warn(
"node_to_link() is deprecated. Use the Block/codec API instead.",
DeprecationWarning,
stacklevel=2,
)
# ...
```

### Related

- Files: `dag/__init__.py`, `dag/dag.py`, `dag/utils.py`

Start by reading dag/__init__.py, dag/dag.py, and dag/utils.py to understand the existing exports and node_to_link() dependency. Resolve whether the project should export Node and Link or deprecate node_to_link(); done means the chosen API direction is implemented consistently and the affected behavior is covered by the project’s tests.

Written by the indexing model from the issue text.