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.

docs/DESIGN.md

@@ -2410,6 +2410,22 @@ are real surface forms.
 { "t": "assign",
   "name": "<id>",
   "value": Term }
+
+// Iter it.1: named loop head. `binders` declares one or more
+// lexically-scoped, typed, initialised binders (initialised in
+// order); `body` is a single Term in scope of all binders. `recur`
+// re-enters the lexically nearest enclosing loop, rebinding the
+// binders positionally; it must be in tail position of that loop's
+// body. The loop's static type is `body`'s static type.
+{ "t": "loop",
+  "binders": [ { "name": "<id>", "type": Type, "init": Term }, ... ],
+  "body": Term }
+
+// Iter it.1: backward jump to the lexically nearest enclosing loop;
+// tail-position-only; binds that loop's binders. The `args` array is
+// always present in canonical JSON (even when empty).
+{ "t": "recur",
+  "args": [ Term, ... ] }
 ```
 
 In the MVP, `do` is only a direct call to a built-in effect op (no
@@ -2425,6 +2441,15 @@ mut-var into a lambda body is rejected at typecheck via
 `CheckError::MutVarCapturedByLambda` (iter mut.4-tidy). See
 `docs/specs/2026-05-15-mut-local.md`.
 
+`loop`/`recur` are additive as of iter it.1 (2026-05-15): they
+parse, print, prose-project, round-trip, typecheck (binder typing,
+recur arity/type unification, recur tail-position) and codegen
+(loop-header block with one phi per binder, `recur` as a back-edge
+`br`) without removing or modifying the existing `tail-app`/`tail-do`
+paths. The structural-recursion restriction and the `Diverge`
+effect land in it.2; `tail-app`/`tail-do` are retired in it.3. See
+`docs/specs/2026-05-15-iteration-discipline.md`.
+
 **`Literal`**:
 
 ```jsonc