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-target-demo.sx

@@ -0,0 +1,28 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-3")
+    (div
+      (~tw :tokens "flex gap-2")
+      (button
+        :sx-get "/sx/(geography.(hypermedia.(reference.(api.time))))"
+        :sx-target "#ref-target-a"
+        :sx-swap "innerHTML"
+        (~tw :tokens "px-3 py-1 bg-violet-600 text-white rounded text-sm hover:bg-violet-700")
+        "Update Box A")
+      (button
+        :sx-get "/sx/(geography.(hypermedia.(reference.(api.time))))"
+        :sx-target "#ref-target-b"
+        :sx-swap "innerHTML"
+        (~tw :tokens "px-3 py-1 bg-emerald-600 text-white rounded text-sm hover:bg-emerald-700")
+        "Update Box B"))
+    (div
+      (~tw :tokens "grid grid-cols-2 gap-3")
+      (div
+        :id "ref-target-a"
+        (~tw :tokens "p-3 rounded border border-violet-200 bg-violet-50 text-sm text-stone-500")
+        "Box A")
+      (div
+        :id "ref-target-b"
+        (~tw :tokens "p-3 rounded border border-emerald-200 bg-emerald-50 text-sm text-stone-500")
+        "Box B"))))