iter it.1: loop/recur additive — Term::Loop/Term::Recur/LoopBinder end-to-end

Iteration-discipline milestone, 1 of 3. Adds named loop + recur as
strictly-additive first-class AST nodes: parse/print/prose/serde/
round-trip/schema lockstep, typecheck (binder typing + recur
arity/type unification via loop_stack threaded as mut.2's
mut_scope_stack; recur-tail-position via verify_loop_body), codegen
(loop-header + one phi per binder; recur back-edge br with a NEW
parallel block_terminated setter; lambda-boundary loop_frames
save/restore). Four Recur* CheckError variants. Strictly additive:
zero deletions touch tail-app/tail-do or the seven existing
block_terminated sites — this is what makes the destructive it.3
safe. recur synth = fresh metavar (resolves the plan's flagged
Type::unit() risk). loop_counter->55, loop_in_lambda->49, four
negatives fire, tail-app byte-identical, cargo test --workspace
green. Spec fda9b78, plan 7381a42.

crates/ail/tests/codegen_import_map_fallback_pin.rs

@@ -101,6 +101,13 @@ fn synthesised_print_uses_user_module_show_via_fallback() {
                     || contains_xmod_show_var(body)
             }
             Term::Assign { value, .. } => contains_xmod_show_var(value),
+            // Iter it.1: a `Term::Loop` cannot itself host a synth'd
+            // cross-module reference, but recurse defensively.
+            Term::Loop { binders, body } => {
+                binders.iter().any(|b| contains_xmod_show_var(&b.init))
+                    || contains_xmod_show_var(body)
+            }
+            Term::Recur { args } => args.iter().any(contains_xmod_show_var),
             Term::Lit { .. } => false,
         }
     }