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/examples/progress-bar-demo.sx

@@ -0,0 +1,19 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-4")
+    (div
+      :id "progress-target"
+      (~tw :tokens "space-y-3")
+      (div
+        (~tw :tokens "w-full bg-stone-200 rounded-full h-4")
+        (div
+          (~tw :tokens "bg-violet-600 h-4 rounded-full transition-all")
+          :style "width: 0%"))
+      (p (~tw :tokens "text-sm text-stone-500 text-center") "Click start to begin."))
+    (button
+      :sx-post "/sx/(geography.(hypermedia.(example.(api.progress-start))))"
+      :sx-target "#progress-target"
+      :sx-swap "innerHTML"
+      (~tw :tokens "px-4 py-2 bg-violet-600 text-white rounded hover:bg-violet-700 transition-colors text-sm")
+      "Start job")))