Fix island SSR: rename signal special form, remove cek-try swallowing

Root cause: the new conditions system's 'signal' special form shadowed
the reactive 'signal' function. (signal 0) in island bodies raised
'Unhandled condition: 0' instead of creating a signal dict.

Fix: rename condition special form to 'signal-condition' in the CEK
dispatcher. The reactive 'signal' function now works normally.

adapter-html.sx: remove cek-try that swallowed island render errors.
Islands now render directly — errors propagate for debugging.

sx_render.ml: add sx_render_to_html that calls SX adapter via CEK.

Results: 4/5 island SSR tests pass:
- Header island: logo, tagline, styled elements ✓
- Navigation buttons ✓
- Geography content ✓
- Stepper: partially renders (code view OK, ~cssx/tw in heading)

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

hosts/ocaml/lib/sx_render.ml

@@ -97,6 +97,16 @@ let render_to_html_ref : (value -> env -> string) ref =
 
 let render_to_html expr env = !render_to_html_ref expr env
 
+(** Render via the SX adapter (render-to-html from adapter-html.sx).
+    Falls back to the native ref if the SX adapter isn't loaded. *)
+let sx_render_to_html render_env expr eval_env =
+  if env_has render_env "render-to-html" then
+    let fn = env_get render_env "render-to-html" in
+    let result = Sx_ref.cek_call fn (List [expr; Env eval_env]) in
+    match result with String s -> s | RawHTML s -> s | _ -> Sx_runtime.value_to_str result
+  else
+    render_to_html expr eval_env
+
 let render_children children env =
   String.concat "" (List.map (fun c -> render_to_html c env) children)