Name the WPM metadata encryption protocol

[rohenaz]

Summary

• expose the exact WPM [2, "admin metadata encryption"] identifier as WalletProtocolID.walletMetadataEncryption
• reuse the existing private canonical construction while retaining the in-process-only BRC-44 boundary
• reject the value from external JSON and wallet-wire encoding/decoding
• run the existing live ProtoWallet metadata-encryption vector through the named constant

Verification

• focused wallet-identifier and ProtoWallet metadata vector tests: 2 passed
• full swift test: 800 passed
• git diff --check