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/vals-headers-demo.sx

@@ -0,0 +1,36 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-6")
+    (div
+      (~tw :tokens "space-y-2")
+      (h4
+        (~tw :tokens "text-sm font-semibold text-stone-700")
+        "sx-vals — send extra values")
+      (button
+        :sx-get "/sx/(geography.(hypermedia.(example.(api.echo-vals))))"
+        :sx-target "#vals-result"
+        :sx-swap "innerHTML"
+        :sx-vals "{\"source\": \"button\", \"version\": \"2.0\"}"
+        (~tw :tokens "px-4 py-2 bg-violet-600 text-white rounded hover:bg-violet-700 transition-colors text-sm")
+        "Send with vals")
+      (div
+        :id "vals-result"
+        (~tw :tokens "p-3 rounded bg-stone-100 text-sm text-stone-400")
+        "Click to see server-received values."))
+    (div
+      (~tw :tokens "space-y-2")
+      (h4
+        (~tw :tokens "text-sm font-semibold text-stone-700")
+        "sx-headers — send custom headers")
+      (button
+        :sx-get "/sx/(geography.(hypermedia.(example.(api.echo-headers))))"
+        :sx-target "#headers-result"
+        :sx-swap "innerHTML"
+        :sx-headers {:X-Request-Source "demo" :X-Custom-Token "abc123"}
+        (~tw :tokens "px-4 py-2 bg-violet-600 text-white rounded hover:bg-violet-700 transition-colors text-sm")
+        "Send with headers")
+      (div
+        :id "headers-result"
+        (~tw :tokens "p-3 rounded bg-stone-100 text-sm text-stone-400")
+        "Click to see server-received headers."))))