Test runner: SSR fix, cek-try primitive, HTML entity handling

- render-html-island wraps body SSR in cek-try (graceful fallback to
  empty container when island body has DOM/signal code)
- defcomp placeholder pattern: server renders safe placeholder div
  with data-sx-island, browser hydrates the actual island
- cek-try primitive added to both server and browser OCaml kernels
- assert/assert= added to spec/harness.sx for standalone use
- Test source stored in <script type="text/sx-test" data-for="...">
  with HTML entity decoding via host-call replaceAll

Temperature converter: 5 tests embedded in demo page. Test runner
hydrates and finds tests but body render is empty — needs debugging
of the specific construct that silently fails in render-to-dom.

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

spec/harness.sx

@@ -1,3 +1,7 @@
+(define assert (fn (condition msg) (when (not condition) (error (or msg "Assertion failed")))))
+
+(define assert= (fn (actual expected msg) (when (not (equal? actual expected)) (error (or msg (str "Expected " expected ", got " actual))))))
+
 (define default-platform {:current-user (fn () nil) :csrf-token (fn () "test-csrf-token") :app-url (fn (service &rest path) "/mock-app-url") :frag (fn (service comp &rest args) "") :sleep (fn (ms) nil) :local-storage-set (fn (key val) nil) :set-cookie (fn (name val &rest opts) nil) :url-for (fn (endpoint &rest args) "/mock-url") :create-element (fn (tag) nil) :request-path (fn () "/") :config (fn (key) nil) :set-attr (fn (el name val) nil) :set-text (fn (el text) nil) :remove-child (fn (parent child) nil) :fetch (fn (url &rest opts) {:status 200 :body "" :ok true}) :query (fn (service name &rest args) (list)) :add-class (fn (el cls) nil) :get-element (fn (id) nil) :now (fn () 0) :abort (fn (code) nil) :action (fn (service name &rest args) {:ok true}) :remove-class (fn (el cls) nil) :append-child (fn (parent child) nil) :request-arg (fn (name) nil) :emit-dom (fn (op &rest args) nil) :local-storage-get (fn (key) nil) :get-cookie (fn (name) nil)})
 
 (define make-harness :effects () (fn (&key platform) (let ((merged (if (nil? platform) default-platform (merge default-platform platform)))) {:log (list) :platform merged :state {:cookies {} :storage {} :dom nil}})))