Step 18 (part 8): Playground page + compile handler + url_decode fix

sx/sx/hyperscript.sx — _hyperscript playground page at
  /sx/(applications.(hyperscript))
  - compile-result defcomp (SSR compilation display)
  - pipeline documentation (tokenize → parse → compile)
  - example showcases with pre-compiled output
  - sx-post form → handler for interactive compilation

sx/sx/handlers/hyperscript-api.sx — POST handler:
  /sx/(applications.(hyperscript.(api.compile)))
  Accepts source param, returns compiled SX + parse tree HTML
  NOTE: hs-parse returns (do) in server context — JIT/CEK runtime
  issue where parser closures don't evaluate correctly. Works in
  test runner (3127/3127). Investigating separately.

sx_server.ml — url_decode fix: decode + as space in form data
  Standard application/x-www-form-urlencoded uses + for spaces.

Nav: _hyperscript added to Applications section.
Config: handler:hs- prefix added for handler dispatch.

3127/3127 tests, zero regressions.

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

sx/sx/handlers/hyperscript-api.sx

@@ -0,0 +1,37 @@
+;; _hyperscript playground API handler
+;; Compiles hyperscript source and returns the AST + SX output as HTML
+
+(defhandler
+  hs-compile
+  :path "/sx/(applications.(hyperscript.(api.compile)))"
+  :method :post
+  :csrf false
+  :returns "element"
+  (&key source)
+  (if
+    (or (nil? source) (empty? source))
+    (p
+      (~tw :tokens "text-sm text-gray-400 italic")
+      "Enter some hyperscript and click Compile.")
+    (let
+      ((ast (hs-compile source)) (sx (hs-to-sx-from-source source)))
+      (div
+        (~tw :tokens "space-y-4")
+        (div
+          (h4
+            (~tw
+              :tokens "text-xs font-semibold uppercase tracking-wider text-gray-500 mb-2")
+            "Compiled SX")
+          (pre
+            (~tw
+              :tokens "bg-gray-900 text-green-400 p-4 rounded-lg text-sm overflow-x-auto whitespace-pre-wrap font-mono")
+            (sx-serialize sx)))
+        (div
+          (h4
+            (~tw
+              :tokens "text-xs font-semibold uppercase tracking-wider text-gray-500 mb-2")
+            "Parse Tree")
+          (pre
+            (~tw
+              :tokens "bg-gray-900 text-amber-400 p-4 rounded-lg text-sm overflow-x-auto whitespace-pre-wrap font-mono")
+            (sx-serialize ast)))))))