DAG-PB encoder doesn't sort links by Name — violates DAG-PB spec

The DAG-PB specification requires that links be sorted by Name (byte-wise), with named links before unnamed links. `_encode_pb_node()` writes links in the order they appear in the input list without sorting, producing non-canonical output that may not match other implementations.

### Problem

In `dag/codecs/dag_pb.py`:

```python
def _encode_pb_node(node: PBNode) -> bytes:
parts: list[bytes] = []

for link in node.links: # ← Links written in input order
link_bytes = _encode_pb_link(link)
parts.append(_encode_length_delimited(2, link_bytes))

if node.data is not None:
parts.append(_encode_length_delimited(1, node.data))

return b"".join(parts)
```

The [DAG-PB spec](https://ipld.io/specs/codecs/dag-pb/spec/) states:

> "Links MUST be sorted by Name (byte-wise). Links without a Name MUST come after all named links."

This means:

```python
# These two nodes should produce identical bytes, but don't:
node1 = PBNode(links=[
PBLink(hash=cid_b, name="beta"),
PBLink(hash=cid_a, name="alpha"),
])
node2 = PBNode(links=[
PBLink(hash=cid_a, name="alpha"),
PBLink(hash=cid_b, name="beta"),
])

encode(node1) != encode(node2) # BUG: should be equal
```

### Proposed Solution

Sort links before encoding:

```python
def _encode_pb_node(node: PBNode) -> bytes:
parts: list[bytes] = []

# Sort links: named links first (by name, byte-wise), then unnamed links
def link_sort_key(link: PBLink) -> tuple[int, bytes]:
if link.name is not None:
return (0, link.name.encode("utf-8"))
return (1, b"") # Unnamed links come after all named links

sorted_links = sorted(node.links, key=link_sort_key)

for link in sorted_links:
link_bytes = _encode_pb_link(link)
parts.append(_encode_length_delimited(2, link_bytes))

if node.data is not None:
parts.append(_encode_length_delimited(1, node.data))

return b"".join(parts)
```

Add tests verifying that differently-ordered links produce identical output.

### Related

- DAG-PB spec: https://ipld.io/specs/codecs/dag-pb/spec/
- File: `dag/codecs/dag_pb.py`

Start in dag/codecs/dag_pb.py at _encode_pb_node() and review how node.links are emitted. Add coverage for differently ordered named links and unnamed links, then run the relevant DAG-PB codec tests. Done means link order is canonical and equivalent input nodes produce identical encoded bytes.

Written by the indexing model from the issue text.