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/section.sx

@@ -0,0 +1,7 @@
+(defcomp
+  (&key title id &rest children)
+  (section
+    :id id
+    (~tw :tokens "space-y-4")
+    (h2 (~tw :tokens "text-2xl font-semibold text-stone-800") title)
+    children))