tamer: nir::air: Reject Todo* tokens

XIRF->Nir produces `Todo` and `TodoAttr` tokens for many different
things.  The previous approach was to ignore those things so that I could
begin adding portions of packages to the graph and observe how that goes.

But now that I'm starting to be able to compile certain packages that
utilize only small subsets of TAME features, I need to have confidence that
I'm fully parsing them.  This means rejecting tokens that I haven't yet
gotten to.

DEV-13708

tamer/src/nir/air.rs

@@ -25,7 +25,7 @@ use crate::{
     diagnose::{Annotate, Diagnostic},
     fmt::{DisplayWrapper, TtQuote},
     parse::prelude::*,
-    span::{Span, UNKNOWN_SPAN},
+    span::Span,
     sym::{st::raw::U_TRUE, SymbolId},
 };
 use arrayvec::ArrayVec;
@@ -95,7 +95,7 @@ impl ParseState for NirToAir {
         use NirToAir::*;
 
         let _ = tok; // prevent `unused_variables` warning
-        Transition(Ready).ok(Air::Todo(UNKNOWN_SPAN))
+        Transition(Ready).ok(Air::Todo(crate::span::UNKNOWN_SPAN))
     }
 
     #[cfg(feature = "wip-asg-derived-xmli")]
@@ -255,8 +255,13 @@ impl ParseState for NirToAir {
                 Transition(Ready).ok(Air::DocIndepClause(clause))
             }
 
-            (_, Todo(..) | TodoAttr(..)) => {
-                Transition(Ready).ok(Air::Todo(UNKNOWN_SPAN))
+            (_, tok @ (Todo(..) | TodoAttr(..))) => {
+                crate::diagnostic_todo!(
+                    vec![tok.internal_error(
+                        "this token is not yet supported in TAMER"
+                    )],
+                    "unsupported token: {tok}",
+                )
             }
 
             (st, Noop(_)) => Transition(st).incomplete(),