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-encoding-demo.sx

@@ -0,0 +1,22 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-3")
+    (form
+      :sx-post "/sx/(geography.(hypermedia.(reference.(api.upload-name))))"
+      :sx-encoding "multipart/form-data"
+      :sx-target "#ref-encoding-result"
+      :sx-swap "innerHTML"
+      (~tw :tokens "flex gap-2")
+      (input
+        :type "file"
+        :name "file"
+        (~tw :tokens "flex-1 text-sm text-stone-500 file:mr-2 file:px-3 file:py-1 file:rounded file:border-0 file:text-sm file:bg-violet-50 file:text-violet-700"))
+      (button
+        :type "submit"
+        (~tw :tokens "px-4 py-2 bg-violet-600 text-white rounded text-sm hover:bg-violet-700")
+        "Upload"))
+    (div
+      :id "ref-encoding-result"
+      (~tw :tokens "p-3 rounded bg-stone-100 text-stone-400 text-sm")
+      "Select a file and submit.")))