SXC content: docs/examples/home/reference pages + SX testing runner

New sxc/ content tree with 120 page files across docs, examples, home,
and reference demos. sx/sx/testing/ adds page-runner.sx (317L) and
index-runner.sx (394L) — SX-native test runner pages for
browser-based evaluation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

sx/sxc/examples/value-select-demo.sx

@@ -0,0 +1,23 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-3")
+    (div
+      (label (~tw :tokens "block text-sm font-medium text-stone-700 mb-1") "Category")
+      (select
+        :name "category"
+        :sx-get "/sx/(geography.(hypermedia.(example.(api.values))))"
+        :sx-trigger "change"
+        :sx-target "#value-items"
+        :sx-swap "innerHTML"
+        (~tw :tokens "w-full px-3 py-2 border border-stone-300 rounded text-sm focus:outline-none focus:ring-2 focus:ring-violet-500")
+        (option :value "" "Pick a category...")
+        (option :value "Languages" "Languages")
+        (option :value "Frameworks" "Frameworks")
+        (option :value "Databases" "Databases")))
+    (div
+      (label (~tw :tokens "block text-sm font-medium text-stone-700 mb-1") "Item")
+      (select
+        :id "value-items"
+        (~tw :tokens "w-full px-3 py-2 border border-stone-300 rounded text-sm focus:outline-none focus:ring-2 focus:ring-violet-500")
+        (option :value "" "Select a category first...")))))