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/nav-data.sx

@@ -444,7 +444,8 @@
         {:label "SX URLs" :href "/sx/(applications.(sx-urls))"}
         {:label "CSSX" :href "/sx/(applications.(cssx))" :children cssx-nav-items}
         {:label "Protocols" :href "/sx/(applications.(protocol))" :children protocols-nav-items}
-        {:label "sx-pub" :href "/sx/(applications.(sx-pub))"})}
+        {:label "sx-pub" :href "/sx/(applications.(sx-pub))"}
+        {:label "SX Tools" :href "/sx/(applications.(sx-tools))"})}
      {:label "Etc" :href "/sx/(etc)"
       :children (list
         {:label "Essays" :href "/sx/(etc.(essay))" :children essays-nav-items}