DAG-JSON `decode()` doesn't validate `{" /": ...}` namespace strictly

The DAG-JSON spec says any `{" /": ...}` that isn't a CID link or bytes is an error in strict mode. `_restore_from_json()` silently passes through unrecognized patterns.

### Problem

In `dag/codecs/dag_json.py`:

```python
def _restore_from_json(node: Any) -> Any:
if isinstance(node, dict):
if len(node) == 1 and _LINK_KEY in node:
link_value = node[_LINK_KEY]

if isinstance(link_value, str):
return _parse_cid_string(link_value)

if isinstance(link_value, dict) and len(link_value) == 1 and "bytes" in link_value:
return _base64_decode_no_pad(link_value["bytes"])

# ← Unrecognized {"/": ...} patterns are silently ignored!

return {k: _restore_from_json(v) for k, v in node.items()}
# ...
```

This means:

```python
# These should raise errors in strict mode, but don't:
decode(b'{" /": 123}') # Integer value
decode(b'{" /": [1, 2, 3]}') # Array value
decode(b'{" /": {"unknown": "value"}}') # Unrecognized dict
```

### Proposed Solution

Add strict validation:

```python
def _restore_from_json(node: Any, strict: bool = True) -> Any:
if isinstance(node, dict):
if len(node) == 1 and _LINK_KEY in node:
link_value = node[_LINK_KEY]

if isinstance(link_value, str):
return _parse_cid_string(link_value)

if isinstance(link_value, dict) and len(link_value) == 1 and "bytes" in link_value:
return _base64_decode_no_pad(link_value["bytes"])

if strict:
raise ValueError(
f'Invalid DAG-JSON: {{"/": ...}} must be a CID string or '
f'{{"bytes": "..."}}, got {type(link_value).__name__}'
)

return {k: _restore_from_json(v, strict) for k, v in node.items()}
# ...
```

Add a `strict` parameter to `DagJsonCodec.decode()`:

```python
def decode(self, data: bytes, strict: bool = True) -> IPLDNode:
raw = json.loads(data)
return _restore_from_json(raw, strict=strict)
```

### Related

- DAG-JSON spec: https://ipld.io/specs/codecs/dag-json/spec/
- File: `dag/codecs/dag_json.py`

Start in dag/codecs/dag_json.py by reading DagJsonCodec.decode() and _restore_from_json(), then compare their behavior with the DAG-JSON specification linked in the issue. Done means valid CID and bytes links still decode, while unrecognized {"/": ...} values raise in strict mode and recursive values honor the selected mode.

Written by the indexing model from the issue text.