Add OA (OffchainAsset) magic numbers to KnownMagic enum

[JuaniRios]

Problem

The KnownMagic enum in crates/cli/src/meta/magic.rs does not include the OffchainAsset (OA) magic numbers used by the ethgild/SFT tokenization ecosystem for encoding receipt information in OffchainAssetReceiptVault deposits and withdrawals.

These magic numbers are already in use by the gildlab/SFT frontend and the h20.market tokenization UI, but they are not part of the rain-metadata crate, making it impossible to use RainMetaDocumentV1Item for encoding/decoding receipt information.

Missing magic numbers

Number	Name	Purpose
0xffa8e8a9b9cf4a31	OA_SCHEMA	Schema reference for offchain asset data
0xff9fae3cc645f463	OA_HASH_LIST	IPFS hash list for offchain assets
0xffc47a6299e8a911	OA_STRUCTURE	Structured offchain asset data (e.g., receipt information)
0xff8cd2927c8c86cb	OA_TOKEN_IMAGE	Token image metadata
0xffbc38eb14ad2209	OA_TOKEN_CREDENTIAL_LINKS	Token credential links

Additional consideration

The SFT frontend also uses OA_SCHEMA as an additional CBOR map key (beyond the standard 0-4 keys) to embed schema references within a CBOR map item. The current RainMetaDocumentV1Item struct only supports keys 0-4. Supporting this extension would require either:

• Adding an optional schema field to RainMetaDocumentV1Item
• Or a more general mechanism for custom map keys

Context

We are building an issuance bot that interacts with Rain OffchainAssetReceiptVault contracts and need to encode receipt information in the Rain metadata v1 format. Without these magic numbers in the crate, we cannot use the official rain-metadata library and must reimplement the CBOR encoding ourselves.

Related spec PR: rainlanguage/specs#48