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/primitives-table.sx

@@ -0,0 +1,14 @@
+(defcomp
+  (&key category primitives)
+  (div
+    (~tw :tokens "space-y-2")
+    (h4 (~tw :tokens "text-lg font-semibold text-stone-700") category)
+    (div
+      (~tw :tokens "flex flex-wrap gap-2")
+      (map
+        (fn
+          (p)
+          (span
+            (~tw :tokens "inline-block px-2 py-1 rounded bg-stone-100 font-mono text-sm text-stone-700")
+            p))
+        primitives))))