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/tab-btn.sx

@@ -0,0 +1,14 @@
+(defcomp
+  (&key tab label active)
+  (button
+    :sx-get (str "/sx/(geography.(hypermedia.(example.(api.(tabs." tab ")))))")
+    :sx-target "#tab-content"
+    :sx-swap "innerHTML"
+    :sx-push-url (str "/sx/(geography.(hypermedia.(example.tabs)))?tab=" tab)
+    :class (str
+      "px-4 py-2 text-sm font-medium border-b-2 -mb-px transition-colors "
+      (if
+        (= active "true")
+        "border-violet-600 text-violet-600"
+        "border-transparent text-stone-500 hover:text-stone-700"))
+    label))