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/tabs-demo.sx

@@ -0,0 +1,19 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-0")
+    (div
+      (~tw :tokens "flex border-b border-stone-200")
+      :id "tab-buttons"
+      (~examples/tab-btn :tab "tab1" :label "Overview" :active "true")
+      (~examples/tab-btn :tab "tab2" :label "Details" :active "false")
+      (~examples/tab-btn :tab "tab3" :label "History" :active "false"))
+    (div
+      :id "tab-content"
+      (~tw :tokens "p-4 border border-t-0 border-stone-200 rounded-b")
+      (p
+        (~tw :tokens "text-stone-700")
+        "Welcome to the Overview tab. This content is loaded by default.")
+      (p
+        (~tw :tokens "text-stone-500 text-sm mt-2")
+        "Click the tabs above to navigate. Watch the browser URL update."))))