AZIP-21: Rename Fee Juice to AZTEC

[rolldavid]

Summary

Proposes renaming the protocol's enshrined fee-payment asset from Fee Juice to AZTEC, along with every identifier derived from it (~1,527 occurrences across ~249 files in aztec-packages). "Fee Juice" does not convey what the asset is: the in-protocol representation of the $AZTEC ERC20 token, bridged from L1, that is spent to pay transaction fees.

Naming scheme

Two registers, as requested:

• Public API → AZTEC — the Noir contract (AZTEC::at(address)), SDK classes (AZTECPaymentMethod, AZTECPaymentMethodWithClaim), getters (getAZTEC), and ProtocolContractAddress.AZTEC.
• Internals → aztec_fee_payment_token — module/file/directory names, storage, protocol constants (AZTEC_FEE_PAYMENT_TOKEN_ADDRESS), and local variables.

Why it's safe

The change is purely lexical. The canonical address (5), the balances storage slot (1), the storage layout, the L1↔L2 bridge mechanics, and all wire/serialization formats are unchanged — pinned by a build-time assertion in the Test Cases. It therefore needs no new rollup version and carries no on-chain payload / no AZUP execution: it ships as source PRs against aztec-packages in a normal release.

Notes for the editor

• Number bumped from 20 to 21 at reviewer request, to reserve the AZIP-20 namespace for a future token standard.
• Category is Core because it touches the enshrined protocol contract and protocol constants, but it is a naming-only change with no behavioral/economic impact — open to reclassification.

[critesjosh]

@rolldavid can we bump to AZIP-21 to reserve the AZIP-20 namespace for a future token standard?