feat: add managed_core_account_set_transaction_label FFI function

Add mutable access to `FFIManagedCoreAccount` via `inner_mut()` and
expose a function to set or clear transaction labels by txid.

Also add comprehensive tests for:
- `managed_core_account_get_transactions` with real transaction data
- `managed_core_account_free_transactions` verifying no crash on free
- Label round-trip (set, read back, clear, verify cleared)
- Error cases (null account, missing txid)

Author: xdustinface

key-wallet-ffi/FFI_API.md

@@ -4,7 +4,7 @@ This document provides a comprehensive reference for all FFI (Foreign Function I
 
 **Auto-generated**: This documentation is automatically generated from the source code. Do not edit manually.
 
-**Total Functions**: 260
+**Total Functions**: 261
 
 ## Table of Contents
 
@@ -140,7 +140,7 @@ Functions: 64
 
 ### Account Management
 
-Functions: 109
+Functions: 110
 
 | Function | Description | Module |
 |----------|-------------|--------|
@@ -243,6 +243,7 @@ Functions: 109
 | `managed_core_account_get_transaction_count` | Get the number of transactions in a managed account  # Safety  - `account`... | managed_account |
 | `managed_core_account_get_transactions` | Get all transactions from a managed account  Returns an array of... | managed_account |
 | `managed_core_account_get_utxo_count` | Get the number of UTXOs in a managed account  # Safety  - `account` must be... | managed_account |
+| `managed_core_account_set_transaction_label` | Set a label on a transaction record  # Safety  - `account` must be a valid,... | managed_account |
 | `managed_platform_account_free` | Free a managed platform account handle  # Safety  - `account` must be a... | managed_account |
 | `managed_platform_account_get_account_index` | Get the account index of a managed platform account  # Safety  - `account`... | managed_account |
 | `managed_platform_account_get_address_pool` | Get the address pool from a managed platform account  Platform accounts only... | managed_account |
@@ -3245,6 +3246,22 @@ Get the number of UTXOs in a managed account  # Safety  - `account` must be a va
 
 ---
 
+#### `managed_core_account_set_transaction_label`
+
+```c
+managed_core_account_set_transaction_label(account: *mut FFIManagedCoreAccount, txid: *const u8, label: *const c_char, error: *mut FFIError,) -> bool
+```
+
+**Description:**
+Set a label on a transaction record  # Safety  - `account` must be a valid, unique pointer to an FFIManagedCoreAccount - `txid` must be a valid pointer to a 32-byte transaction ID - `label` must be a valid null-terminated UTF-8 string, or null to clear the label - `error` must be a valid pointer to an FFIError
+
+**Safety:**
+- `account` must be a valid, unique pointer to an FFIManagedCoreAccount - `txid` must be a valid pointer to a 32-byte transaction ID - `label` must be a valid null-terminated UTF-8 string, or null to clear the label - `error` must be a valid pointer to an FFIError
+
+**Module:** `managed_account`
+
+---
+
 #### `managed_platform_account_free`
 
 ```c