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-header-retarget-demo.sx

@@ -0,0 +1,26 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-3")
+    (button
+      :sx-get "/sx/(geography.(hypermedia.(reference.(api.retarget))))"
+      :sx-target "#ref-hdr-retarget-main"
+      :sx-swap "innerHTML"
+      (~tw :tokens "px-4 py-2 bg-violet-600 text-white rounded hover:bg-violet-700 transition-colors text-sm")
+      "Load (server retargets)")
+    (div
+      (~tw :tokens "grid grid-cols-2 gap-3")
+      (div
+        (~tw :tokens "rounded border border-stone-200 p-3")
+        (div (~tw :tokens "text-xs text-stone-400 mb-1") "Original target")
+        (div
+          :id "ref-hdr-retarget-main"
+          (~tw :tokens "text-sm text-stone-500")
+          "Waiting..."))
+      (div
+        (~tw :tokens "rounded border border-stone-200 p-3")
+        (div (~tw :tokens "text-xs text-stone-400 mb-1") "Retarget destination")
+        (div
+          :id "ref-hdr-retarget-alt"
+          (~tw :tokens "text-sm text-stone-500")
+          "Waiting...")))))