feat(dagcid): disallow libp2p-key CIDs in DagCid

[Arlodotexe]

This PR implements a guard in DagCid to throw when attempting to store a libp2p-key (IPNS) CID as a DagCid link. IPLD DAG links must be immutable; IPNS/libp2p-key CIDs are mutable names.

• Adds validation in DagCid.Value setter and explicit cast from Cid to DagCid to throw ArgumentException for libp2p-key.
• Adds unit tests in test/DagCidTest.cs covering both valid and invalid paths.

Closes #43.

[Copilot]

Pull Request Overview

This PR adds validation to prevent libp2p-key CIDs from being stored as DagCid links, enforcing IPLD's immutability requirement by rejecting mutable IPNS addresses.

• Adds validation in DagCid.Value setter to throw ArgumentException for libp2p-key content type
• Updates explicit cast operator from Cid to DagCid with same validation logic
• Comprehensive unit test coverage for both valid operations and error conditions

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/DagCid.cs	Implements libp2p-key validation in Value setter and explicit cast operator with detailed error messages
test/DagCidTest.cs	Adds comprehensive test coverage for valid CID operations and libp2p-key rejection scenarios

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