Add Confidential Note Fungible Token ConcurrentSupply extension

🧐 Motivation

Updating _encSupply in-circuit pins the old ciphertext into every mint/burn transcript, so two concurrent mints reject each other (concurrency.md §8.1). Issuance throughput is then one op per block.

📝 Details

contracts/src/token/extensions/ConfidentialNoteFungibleTokenConcurrentSupply.compact

• Same confidential, attestable supply as PrivateSupply, but accounting goes through an ElGamalDeltaInbox.
• Hot path _addMinted / _addBurned only APPENDS an encrypted delta keyed by the writer's own randomness — reads no ledger value, so it commutes with everything.
• A separate permissionless fold absorbs the backlog into _encSupply; attestSupply proves the inbox empty in-circuit so the attested total cannot miss pending deltas.
• Demonstrated by the ConcurrentConfidentialNoteFungibleToken preset.

Landed in #679 (draft). Open decision: re-basing the Regulated preset onto it — its emission policy must credit the inbox inside emitOutput.

Start with contracts/src/token/extensions/ConfidentialNoteFungibleTokenConcurrentSupply.compact and compare its design with concurrency.md §8.1 and the ConcurrentConfidentialNoteFungibleToken preset. Review the draft work in #679, then resolve whether the Regulated preset should be re-based and how its emission policy relates to emitOutput; done means that decision and the extension behavior are settled.