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.

examples/loop_counter.ail

@@ -0,0 +1,12 @@
+(module loop_counter
+
+  (fn main
+    (doc "Iter it.1 — sum 1..10 via an accumulator loop. Expected stdout: 55.")
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body
+      (app print
+        (loop ((var acc (con Int) 0) (var i (con Int) 1))
+          (if (app > i 10)
+            acc
+            (recur (app + acc i) (app + i 1))))))))