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/attr-row.sx

@@ -0,0 +1,25 @@
+(defcomp
+  (&key attr description exists href)
+  (tr
+    (~tw :tokens "border-b border-stone-100")
+    (td
+      (~tw :tokens "px-3 py-2 font-mono text-sm whitespace-nowrap")
+      (if
+        href
+        (a
+          :href href
+          :sx-get href
+          :sx-target "#sx-content"
+          :sx-select "#sx-content"
+          :sx-swap "outerHTML"
+          :sx-push-url "true"
+          (~tw :tokens "text-violet-700 hover:text-violet-900 underline")
+          attr)
+        (span (~tw :tokens "text-violet-700") attr)))
+    (td (~tw :tokens "px-3 py-2 text-stone-700 text-sm") description)
+    (td
+      (~tw :tokens "px-3 py-2 text-center")
+      (if
+        exists
+        (span (~tw :tokens "text-emerald-600 text-sm") "yes")
+        (span (~tw :tokens "text-stone-400 text-sm italic") "not yet")))))