Add SSE, response headers, view transitions, and 5 new sx attributes

Implement missing SxEngine features:
- SSE (sx-sse, sx-sse-swap) with EventSource management and auto-cleanup
- Response headers: SX-Trigger, SX-Retarget, SX-Reswap, SX-Redirect,
  SX-Refresh, SX-Location, SX-Replace-Url, SX-Trigger-After-Swap/Settle
- View Transitions API: transition:true swap modifier + global config
- every:<time> trigger for polling (setInterval)
- sx-replace-url (replaceState instead of pushState)
- sx-disabled-elt (disable elements during request)
- sx-prompt (window.prompt, value sent as SX-Prompt header)
- sx-params (filter form parameters: *, none, not x,y, x,y)

Adds docs (ATTR_DETAILS, BEHAVIOR_ATTRS, headers, events), demo
components in reference.sx, API endpoints (prompt-echo, sse-time),
and 27 new unit tests for engine logic.

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

sx/bp/pages/routes.py

@@ -882,4 +882,23 @@ def register(url_prefix: str = "/") -> Blueprint:
         oob = _ref_wire("sx-retry", sx_src)
         return sx_response(f'(<> {sx_src} {oob})')
 
+    @bp.get("/reference/api/prompt-echo")
+    async def ref_prompt_echo():
+        from shared.sx.helpers import sx_response
+        name = request.headers.get("SX-Prompt", "anonymous")
+        sx_src = f'(span :class "text-stone-800 text-sm" "Hello, " (strong "{name}") "!")'
+        oob = _ref_wire("sx-prompt", sx_src)
+        return sx_response(f'(<> {sx_src} {oob})')
+
+    @bp.get("/reference/api/sse-time")
+    async def ref_sse_time():
+        async def generate():
+            for _ in range(30):  # stream for 60 seconds max
+                now = datetime.now().strftime("%H:%M:%S")
+                sx_src = f'(span :class "text-emerald-700 font-mono text-sm" "Server time: {now}")'
+                yield f"event: time\ndata: {sx_src}\n\n"
+                await asyncio.sleep(2)
+        return Response(generate(), content_type="text/event-stream",
+                       headers={"Cache-Control": "no-cache", "X-Accel-Buffering": "no"})
+
     return bp