Add inline test runner for temperature converter demo

Temperature converter tests (6 tests): initial value, computed
fahrenheit derivation, +5/-5 click handlers, reactive propagation,
multiple click accumulation.

New components:
- sx/sx/reactive-islands/test-runner.sx — reusable defisland that
  parses test source, runs defsuite/deftest forms via cek-eval, and
  displays pass/fail results with re-run button
- sx/sx/reactive-islands/test-temperature.sx — standalone test file

Added cek-try primitive to both browser (sx_browser.ml) and server
(sx_server.ml) for safe test execution with error catching.

Browser bundle now includes harness files (harness.sx,
harness-reactive.sx, harness-web.sx) for inline test execution.

Known: SSR renders test runner body instead of placeholder, causing
arity error on complex str expressions. Needs island SSR handling fix.

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

hosts/ocaml/bin/sx_server.ml

@@ -385,6 +385,16 @@ let setup_evaluator_bridge env =
     | [fn_val; List call_args] ->
       Sx_ref.cek_call fn_val (List call_args)
     | _ -> raise (Eval_error "call-lambda: expected (fn args env?)"));
+  bind "cek-try" (fun args ->
+    match args with
+    | [thunk; handler] ->
+      (try Sx_ref.cek_call thunk Nil
+       with Eval_error msg -> Sx_ref.cek_call handler (String msg))
+    | [thunk] ->
+      (try let r = Sx_ref.cek_call thunk Nil in
+        List [Symbol "ok"; r]
+       with Eval_error msg -> List [Symbol "error"; String msg])
+    | _ -> Nil);
   bind "cek-call" (fun args ->
     match args with
     | [fn_val; List call_args] -> Sx_ref.cek_call fn_val (List call_args)