Add Confidential Note Fungible Token Allowlist extension

🧐 Motivation

KYC on a pool with hidden spenders cannot use a Set lookup — the lookup itself would disclose who is transacting, defeating the sender privacy the model is for.

📝 Details

contracts/src/token/extensions/ConfidentialNoteFungibleTokenAllowlist.compact

• Membership is proven in zero knowledge against a Merkle tree of identity leaves: an observer learns only "an allowed party", never which one.
• _assertAllowed is wired at the spend chokepoints; admin _addAllowed / _removeAllowed are gated by the consumer.
• Removal tombstones the leaf against the current root, so revocation is instant.
• Imports no token module.

Landed in #679 (draft). Follow-up: admin-vs-spend liveness — every _addAllowed aborts in-flight KYC spends via the current-root pin; evaluate HistoricMerkleTree + resetHistory on removal so additions stop hurting while removals keep instant revocation.

Start with contracts/src/token/extensions/ConfidentialNoteFungibleTokenAllowlist.compact and the draft in #679. Trace the current-root pinning used by _addAllowed and removal tombstones, then evaluate HistoricMerkleTree with resetHistory. Done means additions no longer abort in-flight KYC spends while removals retain instant revocation.