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/examples/infinite-scroll-demo.sx

@@ -0,0 +1,22 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "h-64 overflow-y-auto border border-stone-200 rounded")
+    :id "scroll-container"
+    (div
+      :id "scroll-items"
+      (map-indexed
+        (fn
+          (i item)
+          (div
+            (~tw :tokens "px-4 py-3 border-b border-stone-100 text-sm text-stone-700")
+            (str "Item " (+ i 1) " — loaded with the page")))
+        (list 1 2 3 4 5))
+      (div
+        :id "scroll-sentinel"
+        :sx-get "/sx/(geography.(hypermedia.(example.(api.scroll))))?page=2"
+        :sx-trigger "intersect once"
+        :sx-target "#scroll-items"
+        :sx-swap "beforeend"
+        (~tw :tokens "p-3 text-center text-stone-400 text-sm")
+        "Loading more..."))))