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>

Common/Client/go/wshub/ringbuf_test.go

@@ -120,7 +120,9 @@ func TestActiveWindowSecTakesTheWidestClientWindow(t *testing.T) {
 }
 
 // An armed trigger owns the window: its pre-window has to be in the buffer
-// before the trigger fires or the capture has nothing to back-fill from.
+// before the trigger fires or the capture has nothing to back-fill from. The
+// buffers must reach back past the window itself, because the capture is read
+// out a margin and a tick after its last sample lands.
 func TestActiveWindowSecPrefersTheArmedTrigger(t *testing.T) {
 	h := NewHub()
 	c := &wsClient{}
@@ -128,8 +130,9 @@ func TestActiveWindowSecPrefersTheArmedTrigger(t *testing.T) {
 	h.clients[c] = true
 	h.trigger.SetConfig(trigConfig{signalKey: "s1:sig", windowSec: 45, mode: "normal"})
 
-	if got := h.activeWindowSec(); got != 45 {
-		t.Fatalf("activeWindowSec = %v, want the trigger's 45", got)
+	if got := h.activeWindowSec(); got != 45+captureLagSec {
+		t.Fatalf("activeWindowSec = %v, want the trigger's 45 plus the %v harvest lag",
+			got, captureLagSec)
 	}
 }