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/loop_counter.ail

@@ -1,8 +1,8 @@
 (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)))
+    (doc "Iter it.1 — sum 1..10 via an accumulator loop. Iter it.2: loop-bearing ⇒ !Diverge added (D2). Expected stdout: 55.")
+    (type (fn-type (params) (ret (con Unit)) (effects IO Diverge)))
     (params)
     (body
       (app print