Add `account_id` MASM helper to test for zero

[PhilippGackstatter]

We have a few places where we check whether an account ID is zero. It'd be nice to have a small helper that implements this for more clarity to replace code like dup.1 eq.0 dup.1 eq.0 and with exec.account_id::testz.

So, we should add helpers in the account_id module that tests whether an account ID is zero, one that consumes the inputs and one that doesn't (mirroring miden::core::word::{eqz, testz}):

#! Returns a boolean indicating whether the given account_id limbs are both zero.
#!
#! Inputs:  [account_id_suffix, account_id_prefix]
#! Outputs: [is_zero, account_id_suffix, account_id_prefix]
#!
#! Where:
#! - account_id_{suffix,prefix} are the suffix and prefix felts of an account ID.
#! - is_zero is a boolean indicating whether the account ID is zero.
pub proc testz

#! Returns a boolean indicating whether the given account_id limbs are both zero.
#!
#! Inputs:  [account_id_suffix, account_id_prefix]
#! Outputs: [is_zero]
#!
#! Where:
#! - account_id_{suffix,prefix} are the suffix and prefix felts of an account ID.
#! - is_zero is a boolean indicating whether the account ID is zero.
pub proc eqz

Maybe it makes sense to rename is_equal to eq for consistency?

Then update the following places in MASM code where account IDs are checked for being zero:

Direct Zero Checks (manual dup + eq.0 pattern)

(This includes PR #2712)

1. crates/miden-protocol/asm/kernels/transaction/api.masm:1502
   dup.1 eq.0 dup.1 eq.0 and not assert.err=ERR_FOREIGN_ACCOUNT_ID_IS_ZERO
2. Asserts foreign account ID is not zero by testing both elements.
3. crates/miden-standards/asm/standards/access/ownable2step.masm:253
   dup.1 eq.0 dup.1 eq.0 and
4. Checks if nominated owner is zero (cancellation of ownership transfer).
5. crates/miden-standards/asm/standards/access/ownable2step.masm:297
   dup.1 eq.0 dup.1 eq.0 and
6. Checks if nominated owner is zero to verify a transfer exists.

Using account_id::is_equal with pushed zeros

4. crates/miden-protocol/asm/kernels/transaction/lib/account.masm:1831
   dup.1 dup.1 push.0 push.0 exec.account_id::is_equal
5. Compares account ID with (0, 0) to detect uninitialized memory blocks when loading foreign
   accounts.
6. crates/miden-agglayer/asm/agglayer/bridge/bridge_config.masm:219-222
   dup.3 dup.3 push.0.0 exec.account_id::is_equal
   assertz.err=ERR_TOKEN_NOT_REGISTERED
7. Validates that a faucet ID from registry is not zero (token is registered).

[AlgofootPrint]

Hi, I'd like to work on this! Here's my plan:

New procedures in account_id module:

• testz — [suffix, prefix] → [is_zero, suffix, prefix] (non-consuming)
• eqz — [suffix, prefix] → [is_zero] (consuming)
  Both mirroring miden::core::word::{testz, eqz}

Call sites to update:

• crates/miden-protocol/asm/kernels/transaction/api.masm:1502 —replace dup.1 eq.0 dup.1 eq.0 and not assert with
  exec.account_id::testz not assert
• crates/miden-standards/asm/standards/access/ownable2step.masm:253 —replace manual dup+eq.0 pattern
• crates/miden-standards/asm/standards/access/ownable2step.masm:297 — same
• crates/miden-protocol/asm/kernels/transaction/lib/account.masm:1831— replace push.0 push.0 exec.account_id::is_equal with exec.account_id::eqz
• crates/miden-agglayer/asm/agglayer/bridge/bridge_config.masm:219-222— same

I'll also rename is_equal → eq for consistency with core library conventions if that sounds good to maintainers.

Happy to open a PR once assigned — I'll target the next branch.