Add header and event detail pages, fix copyright, rename essay

- Detail pages for all 18 HTTP headers with descriptions, example usage,
  direction badges (request/response/both), and live demos for SX-Prompt,
  SX-Trigger, SX-Retarget
- Detail pages for all 10 DOM events with descriptions, example usage,
  and live demos for beforeRequest, afterSettle, responseError,
  validationFailed
- Header and event table rows now link to their detail pages
- Fix copyright symbol on home page (was literal \u00a9, now actual ©)
- Rename "Godel, Escher, Bach" essay to "Strange Loops" with updated summary
- Remove duplicate script injection from bootstrapper page

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/bp/pages/routes.py

@@ -901,4 +901,30 @@ def register(url_prefix: str = "/") -> Blueprint:
         return Response(generate(), content_type="text/event-stream",
                        headers={"Cache-Control": "no-cache", "X-Accel-Buffering": "no"})
 
+    # --- Header demos ---
+
+    @bp.get("/reference/api/trigger-event")
+    async def ref_trigger_event():
+        from shared.sx.helpers import sx_response
+        now = datetime.now().strftime("%H:%M:%S")
+        sx_src = f'(span :class "text-stone-800 text-sm" "Loaded at " (strong "{now}") " — check the border!")'
+        resp = sx_response(sx_src)
+        resp.headers["SX-Trigger"] = "showNotice"
+        return resp
+
+    @bp.get("/reference/api/retarget")
+    async def ref_retarget():
+        from shared.sx.helpers import sx_response
+        now = datetime.now().strftime("%H:%M:%S")
+        sx_src = f'(span :class "text-violet-700 text-sm" "Retargeted at " (strong "{now}"))'
+        resp = sx_response(sx_src)
+        resp.headers["SX-Retarget"] = "#ref-hdr-retarget-alt"
+        return resp
+
+    # --- Event demos ---
+
+    @bp.get("/reference/api/error-500")
+    async def ref_error_500():
+        return Response("Server error", status=500, content_type="text/plain")
+
     return bp