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-event-response-error-demo.sx

@@ -0,0 +1,20 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-3")
+    (button
+      :sx-get "/sx/(geography.(hypermedia.(reference.(api.error-500))))"
+      :sx-target "#ref-evt-err-result"
+      :sx-swap "innerHTML"
+      :sx-on:sx:responseError "var s=document.getElementById('ref-evt-err-status'); s.style.display='block'; s.textContent='Error ' + (event.detail ? event.detail.status || '?' : '?') + ' received'"
+      (~tw :tokens "px-4 py-2 bg-violet-600 text-white rounded hover:bg-violet-700 transition-colors text-sm")
+      "Call failing endpoint")
+    (div
+      :id "ref-evt-err-status"
+      (~tw :tokens "p-2 rounded bg-red-50 text-red-600 text-sm")
+      :style "display: none"
+      "")
+    (div
+      :id "ref-evt-err-result"
+      (~tw :tokens "p-3 rounded bg-stone-100 text-stone-400 text-sm")
+      "Click to trigger an error — the sx:responseError event fires.")))