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-dom.sx

@@ -1146,7 +1146,7 @@
       (let ((initial (cek-run
                        (make-cek-state expr env
                          (list (make-reactive-reset-frame env update-fn true))))))
-        (invoke update-fn initial)))))
+        (cek-call update-fn (list initial))))))
 
 
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