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-optimistic-demo.sx

@@ -0,0 +1,29 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-2")
+    (div
+      :id "ref-opt-item-1"
+      (~tw :tokens "flex items-center justify-between p-2 border border-stone-200 rounded")
+      (span (~tw :tokens "text-sm text-stone-700") "Optimistic item A")
+      (button
+        :sx-delete "/sx/(geography.(hypermedia.(reference.(api.(item.opt1)))))"
+        :sx-target "#ref-opt-item-1"
+        :sx-swap "delete"
+        :sx-optimistic "remove"
+        (~tw :tokens "text-red-500 text-sm hover:text-red-700")
+        "Remove"))
+    (div
+      :id "ref-opt-item-2"
+      (~tw :tokens "flex items-center justify-between p-2 border border-stone-200 rounded")
+      (span (~tw :tokens "text-sm text-stone-700") "Optimistic item B")
+      (button
+        :sx-delete "/sx/(geography.(hypermedia.(reference.(api.(item.opt2)))))"
+        :sx-target "#ref-opt-item-2"
+        :sx-swap "delete"
+        :sx-optimistic "remove"
+        (~tw :tokens "text-red-500 text-sm hover:text-red-700")
+        "Remove"))
+    (p
+      (~tw :tokens "text-xs text-stone-400")
+      "Items fade out immediately on click (optimistic), then are removed when the server responds.")))