Add optional Diamond Storage to ERC-8048

- Add Optional Diamond Storage section specifying namespace 'erc8048.onchain.metadata.storage'
- Add Diamond Storage reference implementation with predictable storage locations
- Update pragma to ^0.8.25 in Basic Implementation
- Add security considerations for Diamond Storage (ERC-8042)
- Clarify benefits for cross-chain applications using inclusion proofs
- Add both Basic and Diamond Storage implementation examples

Author: nxt3d

ERCS/erc-8048.md

@@ -54,6 +54,12 @@ event MetadataSet(uint256 indexed tokenId, string indexed indexedKey, string key
 
 This ERC specifies that the key is a string type and the value is bytes type. This provides flexibility for storing any type of data while maintaining an intuitive string-based key interface.
 
+### Optional Diamond Storage
+
+Contracts implementing this ERC MAY use Diamond Storage pattern for predictable storage locations. If implemented, contracts MUST use the namespace ID `"erc8048.onchain.metadata.storage"`.
+
+The Diamond Storage pattern provides predictable storage locations for data, which is useful for cross-chain applications using inclusion proofs. For more details on Diamond Storage, see ERC-8042.
+
 ### Examples
 
 The inspiration for this standard was trustless AI agents. The registry extends ERC-721 by adding getMetadata and setMetadata functions for optional extra on-chain agent metadata.
@@ -89,11 +95,13 @@ This design prioritizes simplicity and flexibility by using a string-key, bytes-
 
 ## Reference Implementation
 
-The interface is defined in the Required Metadata Function and Event section above. Here is a reference implementation:
+The interface is defined in the Required Metadata Function and Event section above. Here are reference implementations:
+
+### Basic Implementation
 
 ```solidity
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity ^0.8.25;
 
 import "./IOnchainMetadata.sol";
 
@@ -116,12 +124,53 @@ contract OnchainMetadataExample is IOnchainMetadata {
 }
 ```
 
+### Diamond Storage Implementation
+
+```solidity
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.20;
+
+import "./IOnchainMetadata.sol";
+
+contract OnchainMetadataDiamondExample is IOnchainMetadata {
+    struct OnchainMetadataStorage {
+        mapping(uint256 tokenId => mapping(string key => bytes value)) metadata;
+    }
+
+    // keccak256("erc8048.onchain.metadata.storage")
+    bytes32 private constant ONCHAIN_METADATA_STORAGE_LOCATION =
+        keccak256("erc8048.onchain.metadata.storage");
+
+    function _getOnchainMetadataStorage() private pure returns (OnchainMetadataStorage storage $) {
+        bytes32 location = ONCHAIN_METADATA_STORAGE_LOCATION;
+        assembly {
+            $.slot := location
+        }
+    }
+
+    function getMetadata(uint256 tokenId, string calldata key) 
+        external view override returns (bytes memory) {
+        OnchainMetadataStorage storage $ = _getOnchainMetadataStorage();
+        return $.metadata[tokenId][key];
+    }
+    
+    function setMetadata(uint256 tokenId, string calldata key, bytes calldata value) 
+        external {
+        OnchainMetadataStorage storage $ = _getOnchainMetadataStorage();
+        $.metadata[tokenId][key] = value;
+        emit MetadataSet(tokenId, key, key, value);
+    }
+}
+```
+
 Implementations should follow the standard ERC-721, ERC-1155, ERC-6909, or ERC-8004 patterns while adding the required metadata function and event.
 
 ## Security Considerations
 
 This ERC is designed to put metadata onchain, providing security benefits through onchain storage.
 
+Implementations that choose to use the optional Diamond Storage pattern should consider the security considerations of ERC-8042. 
+
 ## Copyright
 
 Copyright and related rights waived via [CC0](../LICENSE.md).