Fix component-source SSR override, add SX island tests

component-source from data/helpers.sx was overriding the native
OCaml version. The SX version calls env-get with wrong arity (1 arg
vs required 2), producing empty source. Re-bind the native version
in SSR overrides after file loading.

Note: source code still not visible because highlight function
returns empty — separate issue in the aser rendering pipeline.

Also adds:
- spec/tests/test-reactive-islands.sx — 22 SX-native tests for all
  14 reactive island demos (render + signal logic + DOM)
- tests/node/run-sx-tests.js — Node runner for SX test files
- tests/node/test-reactive-islands.js — 39 Node/happy-dom tests

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

hosts/ocaml/bin/sx_server.ml

@@ -2140,6 +2140,31 @@ let http_mode port =
   let bind name fn = ignore (env_bind env name (NativeFn (name, fn))) in
   bind "effect" (fun _args -> Nil);
   bind "register-in-scope" (fun _args -> Nil);
+  (* Re-bind component-source — data/helpers.sx overrides the native version
+     with an SX version that calls env-get with wrong arity.  Native version
+     uses env_get directly and handles pretty-printing in OCaml. *)
+  bind "component-source" (fun args ->
+    match args with
+    | [String name] ->
+      let lookup = if String.length name > 0 && name.[0] = '~'
+                   then name
+                   else "~" ^ name in
+      (try
+        let comp = env_get env lookup in
+        match comp with
+        | Component c ->
+          let params = List (List.map (fun s -> Symbol s) c.c_params) in
+          let form = List [Symbol "defcomp"; Symbol ("~" ^ c.c_name);
+                           params; c.c_body] in
+          String (pretty_print_value form)
+        | Island c ->
+          let params = List (List.map (fun s -> Symbol s) c.i_params) in
+          let form = List [Symbol "defisland"; Symbol ("~" ^ c.i_name);
+                           params; c.i_body] in
+          String (pretty_print_value form)
+        | _ -> String (";; " ^ name ^ ": not a component")
+      with _ -> String (";; component " ^ name ^ " not found"))
+    | _ -> raise (Eval_error "component-source: expected (name)"));
   let jt0 = Unix.gettimeofday () in
   let count = ref 0 in
   let compiler_names = [