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-include-demo.sx

@@ -0,0 +1,26 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-3")
+    (div
+      (~tw :tokens "flex gap-2 items-end")
+      (div
+        (label (~tw :tokens "block text-xs text-stone-500 mb-1") "Category")
+        (select
+          :id "ref-inc-cat"
+          :name "category"
+          (~tw :tokens "px-3 py-2 border border-stone-300 rounded text-sm")
+          (option :value "all" "All")
+          (option :value "books" "Books")
+          (option :value "tools" "Tools")))
+      (button
+        :sx-get "/sx/(geography.(hypermedia.(reference.(api.echo-vals))))"
+        :sx-include "#ref-inc-cat"
+        :sx-target "#ref-include-result"
+        :sx-swap "innerHTML"
+        (~tw :tokens "px-4 py-2 bg-violet-600 text-white rounded text-sm hover:bg-violet-700")
+        "Filter"))
+    (div
+      :id "ref-include-result"
+      (~tw :tokens "p-3 rounded bg-stone-100 text-stone-400 text-sm")
+      "Click Filter — the select value is included in the request.")))