`Block.encode()` doesn't validate CIDv0 requires dag-pb codec

`Block.encode()` accepts `version=0` with any codec. Per the CID specification, CIDv0 must use the `dag-pb` codec. The code should validate this constraint.

### Problem

In `dag/block.py`:

```python
@classmethod
def encode(cls, *, value, codec, hasher="sha2-256", version=1):
if isinstance(codec, int):
codec = get_codec(codec)

encoded = codec.encode(value)
mh = _multihash.digest(encoded, hasher)
cid = make_cid(version, codec.name, mh.encode())
# ← No validation that version=0 requires codec="dag-pb"
return cls(cid=cid, data=encoded, value=value, codec=codec)
```

This allows creating invalid CIDv0 objects:

```python
# This should raise an error, but doesn't:
block = Block.encode(
value=b"hello",
codec=raw.codec, # raw codec, not dag-pb!
version=0, # CIDv0 requires dag-pb
)
```

### Proposed Solution

Add validation:

```python
@classmethod
def encode(cls, *, value, codec, hasher="sha2-256", version=1):
if isinstance(codec, int):
codec = get_codec(codec)

# Validate CIDv0 constraints
if version == 0 and codec.name != "dag-pb":
raise ValueError(
f"CIDv0 requires dag-pb codec, got {codec.name!r}. "
f"Use version=1 for other codecs."
)

# Validate CIDv0 hasher constraint
if version == 0 and hasher != "sha2-256":
raise ValueError(
f"CIDv0 requires sha2-256 hasher, got {hasher!r}."
)

encoded = codec.encode(value)
mh = _multihash.digest(encoded, hasher)
cid = make_cid(version, codec.name, mh.encode())
return cls(cid=cid, data=encoded, value=value, codec=codec)
```

### Related

- CID spec: https://github.com/multiformats/cid
- File: `dag/block.py`

Start with dag/block.py and the Block.encode entry point shown in the issue. Check how codec names and hashers are represented, then verify that CIDv0 rejects non-dag-pb codecs and non-sha2-256 hashers while valid inputs remain supported. Done means the invalid examples raise the documented errors without changing CIDv1 behavior.

Written by the indexing model from the issue text.