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-data-sx-env-demo.sx

@@ -0,0 +1,10 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-3")
+    (div
+      :data-sx "(div :class \"p-3 bg-emerald-50 rounded\" (h3 :class \"font-semibold text-emerald-800\" title) (p :class \"text-sm text-stone-600\" message))"
+      :data-sx-env "{\"title\": \"Dynamic content\", \"message\": \"Variables passed via data-sx-env are available in the expression.\"}")
+    (p
+      (~tw :tokens "text-xs text-stone-400")
+      "The title and message above come from the data-sx-env JSON.")))