Replace invoke with cek-call in adapters and engine

Completes the invoke→cek-call migration across all spec .sx files:
- adapter-sx.sx: map/filter/for-each in aser wire format
- adapter-dom.sx: island render update-fn
- engine.sx: fetch transform callback
- test-cek-reactive.sx: disposal test

Only async-invoke (adapter-async.sx) remains — separate async pattern.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/sx/ref/adapter-sx.sx

@@ -291,7 +291,7 @@
                      (let ((local (env-merge (lambda-closure f) env)))
                        (env-set! local (first (lambda-params f)) item)
                        (aser (lambda-body f) local))
-                     (invoke f item)))
+                     (cek-call f (list item))))
                  coll))
 
         ;; map-indexed
@@ -304,7 +304,7 @@
                                (env-set! local (first (lambda-params f)) i)
                                (env-set! local (nth (lambda-params f) 1) item)
                                (aser (lambda-body f) local))
-                             (invoke f i item)))
+                             (cek-call f (list i item))))
                          coll))
 
         ;; for-each — evaluate for side effects, aser each body
@@ -317,7 +317,7 @@
                           (let ((local (env-merge (lambda-closure f) env)))
                             (env-set! local (first (lambda-params f)) item)
                             (append! results (aser (lambda-body f) local)))
-                          (invoke f item)))
+                          (cek-call f (list item))))
                       coll)
             (if (empty? results) nil results))