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-after-swap-demo.sx

@@ -0,0 +1,18 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-3")
+    (button
+      :sx-get "/sx/(geography.(hypermedia.(reference.(api.swap-item))))"
+      :sx-target "#ref-evt-as-list"
+      :sx-swap "beforeend"
+      :sx-on:sx:afterSwap "var items = document.querySelectorAll('#ref-evt-as-list > div'); if (items.length) items[items.length-1].scrollIntoView({behavior:'smooth'}); document.getElementById('ref-evt-as-count').textContent = items.length + ' items'"
+      (~tw :tokens "px-4 py-2 bg-violet-600 text-white rounded hover:bg-violet-700 transition-colors text-sm")
+      "Add item (scrolls after swap)")
+    (div :id "ref-evt-as-count" (~tw :tokens "text-sm text-emerald-700") "1 items")
+    (div
+      :id "ref-evt-as-list"
+      (~tw :tokens "p-3 rounded border border-stone-200 space-y-1 max-h-32 overflow-y-auto")
+      (div
+        (~tw :tokens "text-sm text-stone-500")
+        "Items will be appended and scrolled into view."))))