fix(hp48): render edited stack lines while editing

Use the input value for the first line and shift stack lookups down for
subsequent lines during edit mode. Also route Enter through execute() so
editing state is handled consistently before re-rendering.

samples/hp48/index.html

@@ -287,8 +287,11 @@
     }
 
     function getLineValue(line) {
-      if (line === 0 && calc.isEditing) {
-        return calc.inputValue;
+      if (calc.isEditing) {
+        if (line === 0) {
+          return calc.inputValue;
+        }
+        return calc.stack[line - 1];
       }
       return calc.stack[line];
     }
@@ -383,10 +386,7 @@
       try {
         if (event.key === 'Enter') {
           event.preventDefault();
-          if (calc.isEditing) {
-            calc.command('enter');
-          }
-          render();
+          execute('enter');
           return;
         }