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/nav.sx

@@ -0,0 +1,22 @@
+(defcomp
+  (&key items current)
+  (nav
+    (~tw :tokens "flex flex-wrap gap-2 mb-8")
+    (map
+      (fn
+        (item)
+        (a
+          :href (nth item 1)
+          :sx-get (nth item 1)
+          :sx-target "#sx-content"
+          :sx-select "#sx-content"
+          :sx-swap "outerHTML"
+          :sx-push-url "true"
+          :class (str
+            "px-3 py-1.5 rounded text-sm font-medium no-underline "
+            (if
+              (= (nth item 0) current)
+              "bg-violet-100 text-violet-800"
+              "bg-stone-100 text-stone-600 hover:bg-stone-200"))
+          (nth item 0)))
+      items)))