tamer: xir::parse::ele: Remove NT `todo!` for state transition

Everything except for one state was already accounted for.  We can now have
confidence that the parser will never panic due to state transitions (beyond
legitimate error conditions).

There are some `unreachable!`s to contend with still.

DEV-7145

tamer/src/xir/parse/ele.rs

@@ -1028,11 +1028,13 @@ macro_rules! ele_parse {
                             Transition(st).incomplete()
                         },
 
-                        // TODO: Use `is_accepting` guard if we do not utilize
-                        //   exhaustiveness check.
+                        // Note that this does not necessarily represent an
+                        //   accepting state
+                        //     (see `is_accepting`).
                         (
                             st @ (
                                 Expecting_
+                                | NonPreemptableExpecting_
                                 | Closed_(..)
                                 | RecoverEleIgnoreClosed_(..)
                             ),
@@ -1040,8 +1042,6 @@ macro_rules! ele_parse {
                         ) => {
                             Transition(st).dead(tok)
                         }
-
-                        todo => todo!("{todo:?}"),
                     }
                 }