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/retry-demo.sx

@@ -0,0 +1,17 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-4")
+    (button
+      :sx-get "/sx/(geography.(hypermedia.(example.(api.flaky))))"
+      :sx-target "#retry-result"
+      :sx-swap "innerHTML"
+      :sx-retry "exponential:1000:8000"
+      (~tw :tokens "px-4 py-2 bg-violet-600 text-white rounded hover:bg-violet-700 transition-colors text-sm")
+      "Call flaky endpoint")
+    (div
+      :id "retry-result"
+      (~tw :tokens "p-4 rounded border border-stone-200 bg-stone-100 text-center")
+      (p
+        (~tw :tokens "text-stone-400 text-sm")
+        "Endpoint fails twice, succeeds on 3rd attempt."))))