Add Confidential Note Fungible Token core

🧐 Motivation

The mechanism every other piece of the family builds on: a self-contained note-based token with full graph privacy (amounts, sender, recipient hidden), complete and deployable on its own with no extension.

📝 Details

contracts/src/token/ConfidentialNoteFungibleToken.compact

• State is the Zcash split: _commitments: HistoricMerkleTree<32, Bytes<32>> ("what value exists", append-only) and _nullifiers: Set<Bytes<32>> ("what was spent"). Neither reveals amounts or owners.
• cm = H(domain, value, nonce, pk) with a 256-bit nonce for hiding; nf = H(domain, nonce), deliberately omitting any owner secret so seizure can be escrow-free.
• _consumeNote is the heart: proves Merkle membership against a historical root, checks the owner's spend secret, inserts the nullifier (k=14, ~12.2k rows).
• Deliberately role-free and supply-free: ungated _-prefixed blocks (_mint, _mintNote, _transfer, _burn, _consumeNote) plus value conservation; the composing contract supplies all policy.
• Standalone mode returns created notes to the caller for out-of-band handoff; the Delivery extension replaces that with on-chain discovery.

Landed in #679 (draft).