Add Confidential Note Fungible Token Audit extension

🧐 Motivation

A regulated confidential asset needs an auditor who sees everything. Policy-based audit records are evadable: a sender can skip or fake the record. Auditor completeness has to be structural.

📝 Details

contracts/src/token/extensions/ConfidentialNoteFungibleTokenAudit.compact

• The trick: the note's nonce IS the audit channel. _emitAuditedOutput DERIVES the nonce from an ECDH shared secret with the audit key and returns it; the consumer commits exactly that nonce.
• It also pushes an AuditRecord encrypting the output's value and owner to the audit key.
• Because one proof binds the commitment and the record to the same fields, an output the auditor cannot open cannot exist. The public sees nothing.
• Uses crypto/EcdhMask's KDF; imports no token module.

Landed in #679 (draft).

Start with contracts/src/token/extensions/ConfidentialNoteFungibleTokenAudit.compact and read the crypto/EcdhMask KDF usage described in the issue. Compare the draft work in #679, then verify that _emitAuditedOutput derives the nonce from the audit-key ECDH secret, returns it for the consumer commitment, and emits the encrypted AuditRecord bound to the same output fields.