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/test-cek-reactive.sx

@@ -148,7 +148,7 @@
                     true)))))
       ;; Pop scope — call all disposers
       (scope-pop! "sx-island-scope")
-      (for-each (fn (d) (invoke d)) disposers)
+      (for-each (fn (d) (cek-call d nil)) disposers)
       ;; Change signal — no update should fire
       (reset! s 999)
       (assert-equal 0 (len update-calls)))))