Consolidate L2 Withdrawal Events into a Single Enriched Event

### **Description 📹**

On L2 we currently emit two logs per withdrawal:

* `BurnEvent { user, amount, nonce, commitment_hash }`
* `WithdrawalHashAppended { index, commitment_hash, root_hash }`

This forces the sequencer/indexer to correlate two events per withdrawal. We want **one enriched event** that includes both the business data and the MMR/commitment metadata.

### ✅ **Proposed Actions**

**Branch:** `feat/single-withdrawal-event-with-id`

#### 1) Modify L2 MMR append function

**Change** the function that appends a withdrawal commitment (naming may differ; adjust to your module):

```rust
// before (example)
fn append_withdrawal_hash(commitment_hash: felt252) { ... }

// after
fn append_withdrawal_hash(commitment_hash: felt252) -> (new_root: felt252, leaf_index: u256, element_count: u256) {
// append to MMR
// return post-append metadata
}
```

* Return **post-append** metadata:

* `new_root` (mmr.root after append)
* `leaf_index` (position of appended leaf; use u256 if needed)
* `element_count` (total leaves/elements after append)
* **Remove** any `emit WithdrawalHashAppended(...)` in this code path.

#### 2) Update the withdrawal entrypoint to accept an explicit ID and compute enriched commitment

Extend the L2 withdrawal flow (e.g., `burn` function) to accept a **`withdrawal_id: u256`** provided by the sequencer:

```rust
#[external]
fn burn_xzb_for_unlock(
amount_usd: u256, // normalized units
user: ContractAddress,
withdrawal_id: u256, // NEW: sequencer-provided
) {
// ...
}
```

**Commitment hash must include `withdrawal_id`:**

```rust
let commitment_hash = keccak256_packed(
user,
amount_usd,
nonce, // current per-user or global nonce used in claims
block_timestamp()
);
```

#### 3) Replace dual events with a single enriched event

Define a Cairo event struct and **emit once** with all fields.

```rust
#[event]
struct WithdrawalEvent {
#[key] withdrawal_id: u256, // indexed key
#[key] user: ContractAddress, // indexed key
amount_usd: u256,
nonce: u256,
leaf_index: u256, // from append_withdrawal_hash
commitment_hash: felt252, // or split u256 if you store as u256
new_root: felt252,
element_count: u256
}
```

In the withdrawal function:

```rust
let (new_root, leaf_index, element_count) = append_withdrawal_hash(commitment_hash);

emit WithdrawalEvent {
withdrawal_id,
user,
amount_usd,
nonce,
leaf_index,
commitment_hash,
new_root,
element_count
};
```

* **Remove** `emit BurnEvent { ... }`.
* **Remove** `emit WithdrawalHashAppended { ... }`.

#### 4) Tests & listeners

* **Unit tests**

* Update tests to expect **one** `WithdrawalEvent` with the correct field values.
* Assert `leaf_index`, `new_root`, `element_count` reflect **post-append** MMR state.
* Assert `commitment_hash` equals what the verifier logic uses.

### **Acceptance Criteria 🧾**

* Only **one** event (`WithdrawalEvent`) is emitted per withdrawal.
* Event contains: `withdrawal_id`, `amount_usd`, `user`, `nonce`, `leaf_index`, `commitment_hash`, `new_root`, `element_count`.

### **📋 Required Guidelines**

* PR only if assigned
* Complete within **24 hours**
* Include `Close #[issue_id]` in PR description
* Ensure build/tests pass and sequencer logic updated before merge