feat(normalization): Phase 142 P0 - Loop statement-level normalization

Phase 142-loopstmt P0: Statement-level normalization

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Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

apps/tests/phase139_if_only_post_k_return_add_min.hako

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+// Phase 139 P0: if-only post_k return add minimal fixture
+//
+// Purpose: Ensure post_if_post_k.rs delegates return lowering to ReturnValueLowererBox
+// Pattern:
+//   x = 1
+//   flag = 1
+//   if flag == 1 { x = 2 } else { x = 1 }
+//   return x + 2
+// Expected: exit code 4
+//
+// Notes:
+// - Requires explicit else (Phase 129-C contract for post_if_post_k)
+// - Dev-only Normalized shadow path (NYASH_JOINIR_DEV=1 + HAKO_JOINIR_STRICT=1)
+
+static box Main {
+    main() {
+        local x
+        local flag
+        x = 1
+        flag = 1
+        if flag == 1 {
+            x = 2
+        } else {
+            x = 1
+        }
+        return x + 2
+    }
+}
+

apps/tests/phase141_p1_if_only_post_k_return_length_min.hako

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+// Phase 141 P1: ExprLowerer known intrinsic (length0) minimal fixture (if-only)
+//
+// Purpose: Exercise MethodCall lowering via `ReturnValueLowererBox` without loop-boundary merge.
+//
+// Pattern:
+//   s = "abc"
+//   flag = 1
+//   if flag == 1 { s = s } else { s = s }
+//   return s.length()
+//
+// Expected: exit code 3
+// Notes: Dev-only Normalized shadow path (NYASH_JOINIR_DEV=1 + HAKO_JOINIR_STRICT=1)
+
+static box Main {
+    main() {
+        local s
+        local flag
+        s = "abc"
+        flag = 1
+        if flag == 1 {
+            s = s
+        } else {
+            s = s
+        }
+        return s.length()
+    }
+}