Add Confidential Note Fungible Token PrivateSupply extension

🧐 Motivation

"What is the total supply?" has to be answerable without a public counter — a disclosed total leaks every mint and burn as a public delta, which is exactly the leak the note model exists to close.

📝 Details

contracts/src/token/extensions/ConfidentialNoteFungibleTokenPrivateSupply.compact

• _encSupply holds outstanding supply as an exponential-ElGamal ciphertext under _supplyKey; _addMinted / _addBurned update it HOMOMORPHICALLY in the same transaction as the token op, so nobody decrypts to account.
• attestSupply publishes a proof-backed public total when a deployment wants one, without making the running total public.
• Supply-key self-rotation re-encrypts _encSupply under the new key in-proof.
• Uses crypto/ElGamal; imports no token module.

Landed in #679 (draft).