feat: re-export curated code_table constants from package root

[acul71]

Summary

Closes #46.

• Re-export a curated set of commonly used named codec constants from the top-level multicodec package
• Add tests/test_exports.py covering identity, values, and __all__ membership
• Add newsfragment 46.feature.rst

Full set remains available via from multicodec.code_table import ….

Demo

>>> from multicodec import SHA2_256, DAG_CBOR, IP4
>>> SHA2_256, int(SHA2_256), SHA2_256.name
(Code('sha2-256', 0x12), 18, 'sha2-256')
>>> DAG_CBOR, IP4
dag-cbor ip4

Test plan

• make lint / typecheck / test / docs-ci locally
• Newsfragment 46.feature.rst present
• CI green on this PR
• Review requested from @sumanjeet0012

Made with Cursor

[acul71]

@sumanjeet0012 Could you please review this PR when you have a chance? Thanks!