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/docs/code.sx

@@ -0,0 +1,7 @@
+(defcomp
+  (&key src)
+  (div
+    (~tw :tokens "not-prose bg-stone-100 rounded-lg p-5 overflow-x-auto my-6")
+    (pre
+      (~tw :tokens "text-sm leading-relaxed whitespace-pre-wrap break-words font-mono")
+      src)))