iter it.2: structural-guardedness checker + first real Diverge effect

Iteration-discipline milestone, 2 of 3. Strictly additive (nothing
tail-related removed; that is it.3). New whole-body pass
verify_structural_recursion sibling of verify_tail_positions
(DD-1): smaller-set algorithm with implicit candidate inference +
unconstrained accumulators (DD-2, foldl=structural), self/mutual
via inline ADT-family union-find (DD-3), it.2-only tail==false
grandfather. CheckError::NonStructuralRecursion. term_contains_loop
(stops at Term::Lam, DD-4) injects Diverge so existing
UndeclaredEffect enforces it, no new variant; lam-arrow + LetRec
sub-effect sites wired. DESIGN.md Decision 3 synced. Four it.1
loop fixtures gained !Diverge.

Two spec-premise boundary defects surfaced + resolved within the
additive invariant (corpus clean, check not weakened), recorded as
corrected it.3 corpus-migration scope: (1) the "21 tail-app
fixtures" grandfather premise under-counts the corpus —
no-ADT-candidate counter recursions have no structural position to
verify, deferred to it.3; (2) two RC-regression fixtures joined the
spec's transitional tail-app grandfather as the other 20 do (RC==GC
guards verified still green). cargo test --workspace 622/0; 9
acceptance pins non-vacuous. Spec fda9b78, plan bc9f512.

examples/struct_rec_foldl_sum.ail

@@ -0,0 +1,26 @@
+; Iter it.2 positive (spec D1): foldl-shape accumulator. `go` recurses
+; on the Cons-tail `t` at position 0 (structurally guarded) while
+; threading an unconstrained accumulator `acc` at position 1. The
+; accumulator position is never examined by the guardedness check, so
+; this classifies as structural recursion: pure, total, Diverge-free,
+; with NO `tail-app` marker. This is the single most LLM-natural
+; iteration shape and must stay structural (spec D1).
+
+(module struct_rec_foldl_sum
+
+  (data IntList
+    (ctor INil)
+    (ctor ICons (con Int) (con IntList)))
+
+  (fn go
+    (doc "Sum via a foldl-shape accumulator; structural on the tail.")
+    (type
+      (fn-type
+        (params (con IntList) (con Int))
+        (ret (con Int))))
+    (params xs acc)
+    (body
+      (match xs
+        (case (pat-ctor INil) acc)
+        (case (pat-ctor ICons h t)
+          (app go t (app + acc h)))))))