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-pushurl-demo.sx

@@ -0,0 +1,27 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-3")
+    (div
+      (~tw :tokens "flex gap-2")
+      (a
+        :href "/sx/(geography.(hypermedia.(reference-detail.attributes.sx-get)))"
+        :sx-get "/sx/(geography.(hypermedia.(reference-detail.attributes.sx-get)))"
+        :sx-target "#sx-content"
+        :sx-select "#sx-content"
+        :sx-swap "outerHTML"
+        :sx-push-url "true"
+        (~tw :tokens "px-3 py-1 bg-violet-100 text-violet-700 rounded text-sm no-underline hover:bg-violet-200")
+        "sx-get page")
+      (a
+        :href "/sx/(geography.(hypermedia.(reference-detail.attributes.sx-post)))"
+        :sx-get "/sx/(geography.(hypermedia.(reference-detail.attributes.sx-post)))"
+        :sx-target "#sx-content"
+        :sx-select "#sx-content"
+        :sx-swap "outerHTML"
+        :sx-push-url "true"
+        (~tw :tokens "px-3 py-1 bg-violet-100 text-violet-700 rounded text-sm no-underline hover:bg-violet-200")
+        "sx-post page"))
+    (p
+      (~tw :tokens "text-sm text-stone-500")
+      "Click a link — the URL bar updates without a full page reload. Use browser back to return.")))