Iter 16b.1 — local recursive let (no-capture)

Add `(let-rec NAME (params ...) (type ...) (body ...) (in ...))`
as a form-A surface and a `Term::LetRec` AST variant. The 16a
desugar pass lifts each LetRec whose body has no captures from
the enclosing scope to a synthetic top-level fn `<hint>$lr_N`
and substitutes the original name; typecheck and codegen never
see LetRec. Capture detection panics at desugar time, queued
for 16b.2.

Tests: 95 → 99 (+1 e2e local_rec_factorial_demo, +2 desugar
unit, +1 parse unit). The new fixture `examples/local_rec_demo`
runs `fact` at n=1, 3, 5 → prints 1, 6, 120.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/ailang-codegen/src/lib.rs

@@ -1039,6 +1039,12 @@ impl<'a> Emitter<'a> {
                 let _ = self.lower_term(lhs)?;
                 self.lower_term(rhs)
             }
+            Term::LetRec { .. } => {
+                // Iter 16b.1: `Term::LetRec` is eliminated by the
+                // desugar pass before codegen runs, so reaching it
+                // here is a bug.
+                unreachable!("Term::LetRec eliminated by desugar")
+            }
         }
     }
 
@@ -2122,6 +2128,10 @@ impl<'a> Emitter<'a> {
                 Self::collect_captures(lhs, bound, captures, captures_set, builtins, top_level);
                 Self::collect_captures(rhs, bound, captures, captures_set, builtins, top_level);
             }
+            Term::LetRec { .. } => {
+                // Iter 16b.1: eliminated by desugar before codegen.
+                unreachable!("Term::LetRec eliminated by desugar")
+            }
         }
     }
 
@@ -2548,6 +2558,10 @@ impl<'a> Emitter<'a> {
                 }
             }
             Term::Seq { rhs, .. } => self.synth_with_extras(rhs, extras),
+            Term::LetRec { .. } => {
+                // Iter 16b.1: eliminated by desugar before codegen.
+                unreachable!("Term::LetRec eliminated by desugar")
+            }
         }
     }
 }
@@ -2898,6 +2912,11 @@ fn apply_subst_to_term(t: &Term, subst: &BTreeMap<String, Type>) -> Term {
             lhs: Box::new(apply_subst_to_term(lhs, subst)),
             rhs: Box::new(apply_subst_to_term(rhs, subst)),
         },
+        Term::LetRec { .. } => {
+            // Iter 16b.1: eliminated by desugar before any
+            // monomorphisation pass runs.
+            unreachable!("Term::LetRec eliminated by desugar")
+        }
     }
 }