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-event-sse-error-demo.sx

@@ -0,0 +1,20 @@
+(defcomp
+  ()
+  (div
+    (~tw :tokens "space-y-3")
+    (div
+      :sx-sse "/sx/(geography.(hypermedia.(reference.(api.sse-time))))"
+      :sx-sse-swap "time"
+      :sx-swap "innerHTML"
+      :sx-on:sx:sseError "document.getElementById('ref-evt-sseerr-status').textContent = 'Disconnected'; document.getElementById('ref-evt-sseerr-status').className = 'inline-block px-2 py-0.5 rounded text-xs bg-red-100 text-red-700'"
+      :sx-on:sx:sseOpen "document.getElementById('ref-evt-sseerr-status').textContent = 'Connected'; document.getElementById('ref-evt-sseerr-status').className = 'inline-block px-2 py-0.5 rounded text-xs bg-emerald-100 text-emerald-700'"
+      (div
+        (~tw :tokens "flex items-center gap-3")
+        (span
+          :id "ref-evt-sseerr-status"
+          (~tw :tokens "inline-block px-2 py-0.5 rounded text-xs bg-amber-100 text-amber-700")
+          "Connecting...")
+        (span (~tw :tokens "text-sm text-stone-500") "SSE stream")))
+    (p
+      (~tw :tokens "text-xs text-stone-400")
+      "If the SSE connection drops, the badge turns red via sx:sseError.")))