Fix home-stepper paren bugs, harden defisland multi-body, add SX Tools essay

Two paren bugs in home-stepper.sx caused the home page to render blank:

1. Line 222 had one extra ) that prematurely closed the letrec bindings
   list — rebuild-preview and do-back became body expressions instead
   of bindings, making them undefined in scope.

2. Lines 241-308 were outside the let/letrec scope entirely — the outer
   let closed at line 240, so freeze-scope, cookie restore, source
   parsing, and the entire div rendering tree had no access to signals
   or letrec functions.

Also hardens defisland to wrap multi-expression bodies in (begin ...),
matching the Python-side fix from 9f0c541. Both spec/evaluator.sx and
the OCaml transpiled sx_ref.ml are updated.

Adds SX Tools essay under Applications — the revised plan for structural
tree reading/editing tools for .sx files, motivated by this exact bug.

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

spec/evaluator.sx

@@ -734,13 +734,17 @@
 
 (define sf-defisland
   (fn ((args :as list) (env :as dict))
-    ;; (defisland ~name (params) body)
+    ;; (defisland ~name (params) body...)
     ;; Like defcomp but creates an island (reactive component).
     ;; Islands have the same calling convention as components but
     ;; render with a reactive context on the client.
+    ;; Multi-expression bodies are wrapped in (begin ...).
     (let ((name-sym   (first args))
           (params-raw (nth args 1))
-          (body       (last args))
+          (body-exprs (slice args 2))
+          (body       (if (= (len body-exprs) 1)
+                        (first body-exprs)
+                        (cons (make-symbol "begin") body-exprs)))
           (comp-name  (strip-prefix (symbol-name name-sym) "~"))
           (parsed     (parse-comp-params params-raw))
           (params     (first parsed))