`collect_links()` imports private `_walk_links` from `block` module

`utils.py` imports `_walk_links` (a private function) from `block.py`. This cross-module use of a private function is fragile and should be refactored.

### Problem

In `dag/utils.py`:

```python
def collect_links(value: Any) -> list[tuple[str, CID]]:
from .block import _walk_links # ← Imports private function

return list(_walk_links(value, ""))
```

In `dag/block.py`:

```python
def _walk_links(node: Any, prefix: str) -> Iterator[tuple[str, CID]]:
"""Recursively yield (path, cid) from an IPLD data-model value."""
# ...
```

The `_walk_links` function is used by both `Block.links()` and `collect_links()`, but it's marked as private with the `_` prefix.

### Proposed Solution

Move `_walk_links` to `utils.py` and make it public:

```python
# In dag/utils.py:
def walk_links(node: Any, prefix: str = "") -> Iterator[tuple[str, CID]]:
"""Recursively yield (path, cid) from an IPLD data-model value."""
if is_cid(node):
yield (prefix, node)
elif isinstance(node, dict):
for k, v in node.items():
child = f"{prefix}/{k}" if prefix else k
yield from walk_links(v, child)
elif isinstance(node, list):
for i, v in enumerate(node):
child = f"{prefix}/{i}" if prefix else str(i)
yield from walk_links(v, child)

def collect_links(value: Any) -> list[tuple[str, CID]]:
return list(walk_links(value, ""))
```

Update `block.py` to import from `utils.py`:

```python
from .utils import walk_links as _walk_links
```

Export `walk_links` from `__init__.py`.

### Related

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

Read the existing _walk_links implementation and collect_links in dag/utils.py, then inspect Block.links() and the import in dag/block.py. Check dag/__init__.py exports and the related call sites; done means walk_links is public, the private cross-module import is removed, and link collection behavior remains unchanged.

Written by the indexing model from the issue text.