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/echo-result.sx

@@ -0,0 +1,8 @@
+(defcomp
+  (&key label items)
+  (div
+    (~tw :tokens "space-y-1")
+    (p (~tw :tokens "text-stone-800 font-medium") (str "Server received " label ":"))
+    (map
+      (fn (item) (div (~tw :tokens "text-sm text-stone-600 font-mono") item))
+      items)))