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/engine.sx

@@ -530,7 +530,7 @@
       (if (and env new-html (not (empty? new-html)))
         ;; Parse new content as SX and re-evaluate in island scope
         (let ((parsed (parse new-html)))
-          (let ((sx-content (if transform (invoke transform parsed) parsed)))
+          (let ((sx-content (if transform (cek-call transform (list parsed)) parsed)))
             ;; Dispose old reactive bindings in this marsh
             (dispose-marsh-scope old-marsh)
             ;; Evaluate the SX in a new marsh scope — creates new reactive bindings