fix(state-machine): prevent cancel after bills accepted in cash-in

Once bills are stacked in the cash box they cannot be returned.
Cancel in displayingQR now goes to confirmAbandon warning state.
Error state retries to generatingNdebit instead of idle when bills
are present. CANCEL from error only goes to idle if no bills inserted.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/state-machine/src/machine.ts

@@ -431,6 +431,7 @@ export function createATMMachine(
             },
           },
           generatingNdebit: {
+            // No CANCEL here — bills are already in the cash box
             invoke: {
               src: 'generateNdebit',
               input: ({ context }) => context,
@@ -457,8 +458,15 @@ export function createATMMachine(
                 target: 'error',
                 actions: ['setError', 'setPaymentFailed'],
               },
-              TIMEOUT: '#atm.idle',
+              CANCEL: 'confirmAbandon',
-              CANCEL: '#atm.idle',
+              TIMEOUT: 'confirmAbandon',
+            },
+          },
+          confirmAbandon: {
+            // Warning: cash is in the machine, abandoning forfeits it
+            on: {
+              CANCEL: '#atm.idle', // User confirms they want to leave
+              RETRY: 'generatingNdebit', // Go back and try again
             },
           },
           askForReceipt: {
@@ -489,10 +497,20 @@ export function createATMMachine(
             on: {
               RETRY: {
                 guard: 'canRetry',
-                target: 'fetchingRate',
+                target: 'generatingNdebit',
                 actions: 'incrementRetry',
               },
-              CANCEL: '#atm.idle',
+              CANCEL: [
+                {
+                  // No bills inserted: safe to cancel
+                  guard: ({ context }) => context.billsInserted.length === 0,
+                  target: '#atm.idle',
+                },
+                {
+                  // Bills inserted: show abandon warning first
+                  target: 'confirmAbandon',
+                },
+              ],
             },
           },
         },