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-media-demo.sx

@@ -0,0 +1,17 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-3")
+    (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"
+      :sx-media "(min-width: 768px)"
+      (~tw :tokens "inline-block px-4 py-2 bg-violet-600 text-white rounded text-sm no-underline hover:bg-violet-700")
+      "sx navigation (desktop only)")
+    (p
+      (~tw :tokens "text-sm text-stone-500")
+      "On screens narrower than 768px this link uses normal navigation. On wider screens it uses sx.")))