Add Confidential Note Fungible Token Freeze extension

🧐 Motivation

Seizure alone is a race: an authority that has decided to claw back a note loses if the owner spends first. Regulated deployments need to stop the note moving before they act on it.

📝 Details

contracts/src/token/extensions/ConfidentialNoteFungibleTokenFreeze.compact

• Freeze-before-seize: a frozen-nullifier set checked at the owner-spend chokepoint via _assertNotFrozen.
• A note is frozen by its NULLIFIER. Because nf = H(domain, nonce) depends on the nonce alone, an authority armed by the audit trail derives the target's nullifier without consuming the note — freezing is non-destructive and needs no cooperation from the owner.
• Seizure of a frozen note still works; only owner-spend paths are blocked.
• Imports no token module and holds no roles; the consumer gates _freeze / _unfreeze behind its authority.

Landed in #679 (draft). Follow-up: document the authority sequence freeze → finality → seize (freeze only wins if it lands first).

Start by reading contracts/src/token/extensions/ConfidentialNoteFungibleTokenFreeze.compact and the implementation referenced as #679. Document the authority sequence freeze → finality → seize, including that freeze must land before the owner's spend and that seizure remains possible after freezing.