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>

sx/sx/home-stepper.sx

@@ -219,7 +219,7 @@
                  ;; Component expressions handled by lake's reactive render
                  nil))
            (swap! step-idx inc)
-           (update-code-highlight)))))
+           (update-code-highlight))))
        (rebuild-preview (fn (target)
          ;; Rebuild preview in one shot from steps-to-preview (pure SX→DOM)
          (let ((container (get-preview)))
@@ -237,7 +237,7 @@
              (rebuild-preview target)
              (reset! step-idx target)
              (update-code-highlight)
-             (set-cookie "sx-home-stepper" (freeze-to-sx "home-stepper"))))))))
+             (set-cookie "sx-home-stepper" (freeze-to-sx "home-stepper")))))))
       ;; Freeze scope for persistence — same mechanism, cookie storage
       (freeze-scope "home-stepper" (fn ()
         (freeze-signal "step" step-idx)))
@@ -305,6 +305,6 @@
         ;; Live preview — shows partial result up to current step.
         ;; Same SX rendered by server (HTML) and client (DOM).
         (lake :id "home-preview"
-          (steps-to-preview (deref steps) (deref step-idx))))))
+          (steps-to-preview (deref steps) (deref step-idx))))))))