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/reference/ref-headers-demo.sx

@@ -0,0 +1,15 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-3")
+    (button
+      :sx-get "/sx/(geography.(hypermedia.(reference.(api.echo-headers))))"
+      :sx-headers {:X-Request-Source "demo" :X-Custom-Token "abc123"}
+      :sx-target "#ref-headers-result"
+      :sx-swap "innerHTML"
+      (~tw :tokens "px-4 py-2 bg-violet-600 text-white rounded text-sm hover:bg-violet-700")
+      "Send with custom headers")
+    (div
+      :id "ref-headers-result"
+      (~tw :tokens "p-3 rounded bg-stone-100 text-stone-400 text-sm")
+      "Click to see echoed headers.")))