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/browser/sx-platform.js

@@ -255,6 +255,10 @@
       // Boot helpers (platform functions in pure SX)
       "sx/boot-helpers.sx",
       "sx/hypersx.sx",
+      // Test harness (for inline test runners)
+      "sx/harness.sx",
+      "sx/harness-reactive.sx",
+      "sx/harness-web.sx",
       // Web framework
       "sx/engine.sx",
       "sx/orchestration.sx",