feat(webui): sporadic-trigger capture, CSV export and UI rework

Brings the Go hub and web SPA work developed on feature/udpscope onto
main, without the udpscope client itself.

The trigger engine could not capture a sporadic event: it armed on the
live tail only, so a burst shorter than one push window was already past
by the time the FSM looked for it. It now searches the ring history for
the crossing, which also makes a capture reproducible from the same data
rather than dependent on push timing (wshub/trigger.go, ringbuf.go,
history.go).

Adds CSV/JSON export of the visible window (wshub/export.go) and reworks
the SPA: per-signal axis controls, a readable trigger panel, and a fix
for the flicker caused by repainting on every push instead of on a frame
tick (static/app.js, index.html, style.css).

BUFFER_AND_TRIGGER.md documents the ring/decimation/trigger interaction,
which is otherwise only inferable from the three files that implement it.

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

Client/udpstreamer/main.go

@@ -94,6 +94,7 @@ func main() {
 	http.Handle("/", http.FileServer(http.FS(sub)))
 	http.HandleFunc("/ws", hub.HandleWebSocket)
 	http.HandleFunc("/api/zoom", hub.HandleZoom)
+	http.HandleFunc("/api/export", hub.HandleExport)
 	http.HandleFunc("/version", func(w http.ResponseWriter, r *http.Request) {
 		fmt.Fprint(w, buildVersion)
 	})