`multicodec_codes.py` has incomplete hash code definitions

`multicodec_codes.py` only defines 6 hash codes (identity, sha2-256, sha2-512, sha3-256, sha3-512, blake2b-256). Many commonly used hash functions are missing (shake-128, shake-256, keccak-*, blake3, blake2s-*, murmur3-*, etc.).

### Problem

In `dag/multicodec_codes.py`:

```python
SHA2_256_CODE = 0x12
SHA2_512_CODE = 0x13
SHA3_256_CODE = 0x16
SHA3_512_CODE = 0x17
BLAKE2B_256_CODE = 0xB220
IDENTITY_CODE = 0x00
```

Missing codes include:
- `shake-128` (0x18), `shake-256` (0x19)
- `keccak-224` (0x1A), `keccak-256` (0x1B), `keccak-384` (0x1C), `keccak-512` (0x1D)
- `blake3` (0x1E)
- `dbl-sha2-256` (0x56)
- `sha2-224` (0x1013), `sha2-384` (0x20)
- `blake2b-8` through `blake2b-512` (0xB201–0xB240)
- `blake2s-8` through `blake2s-256` (0xB241–0xB260)
- And many more...

### Proposed Solution

Add all hash codes from the multicodec table. Consider importing from `py-multihash` constants instead of duplicating:

```python
# Option 1: Import from py-multihash
from multihash.constants import HASH_CODES

# Option 2: Add missing codes manually
SHAKE_128_CODE = 0x18
SHAKE_256_CODE = 0x19
KECCAK_224_CODE = 0x1A
KECCAK_256_CODE = 0x1B
KECCAK_384_CODE = 0x1C
KECCAK_512_CODE = 0x1D
BLAKE3_CODE = 0x1E
# ... etc
```

### Related

- Multicodec table: https://github.com/multiformats/multicodec/blob/master/table.csv
- File: `dag/multicodec_codes.py`

Start in dag/multicodec_codes.py and compare the existing constants with the linked multicodec table. Check whether py-multihash exposes the needed HASH_CODES before deciding how to represent the missing entries; the work is done when the hash codes requested by the table are available consistently in this module.

Written by the indexing model from the issue text.