security(H4): fix bill escrow race condition

Guard hal:stack-bill and hal:reject-bill IPC handlers against being
called when no bill is in escrow (pendingBillDenomination === null).
Previously, rapid-fire calls could double-accept or misattribute
bill denominations. Now the handlers silently ignore calls when
no bill is pending, preventing the race.

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

apps/machine/electron/main.ts

@@ -248,22 +248,25 @@ ipcMain.handle('hal:disable-validator', () => {
 })
 
 ipcMain.handle('hal:stack-bill', () => {
-  if (halInstance) {
-    const denomination = pendingBillDenomination
-    pendingBillDenomination = null
-    halInstance.stackBill()
-    // Notify renderer that the bill was accepted
-    if (denomination !== null) {
-      mainWindow?.webContents.send('hal:bill-inserted', denomination)
-    }
+  if (!halInstance) return
+  if (pendingBillDenomination === null) {
+    console.warn('[Electron] hal:stack-bill called with no bill in escrow — ignoring')
+    return
   }
+  const denomination = pendingBillDenomination
+  pendingBillDenomination = null
+  halInstance.stackBill()
+  mainWindow?.webContents.send('hal:bill-inserted', denomination)
 })
 
 ipcMain.handle('hal:reject-bill', () => {
-  if (halInstance) {
-    pendingBillDenomination = null
-    halInstance.rejectBill()
+  if (!halInstance) return
+  if (pendingBillDenomination === null) {
+    console.warn('[Electron] hal:reject-bill called with no bill in escrow — ignoring')
+    return
   }
+  pendingBillDenomination = null
+  halInstance.rejectBill()
 })
 
 ipcMain.handle('hal:get-inventory', () => {