feat(tui): truthful-state §2 — last-event clock + unknown-event raw fallback

Implements the load-bearing TUI-requirements §2 hard requirements:

- Last-event clock: "last event Ns ago" always visible in the status bar while
  in-session, updating every frame tick. Turns warn-colored when an active session
  has gone quiet past the stale threshold — the "thinking vs hung" tell. The
  WebSocket connection indicator (●/◌/○) already covers immediate drop visibility.
- Unknown-event raw fallback: applyServer now surfaces any unrecognized,
  session-scoped event type as a raw row in the event stream instead of silently
  dropping it (the frontend must not lie by omission). Recognized-but-unrendered
  types (router.response, protocol_error) get an explicit no-op so the default
  branch means genuinely unknown.

Tests: agoLabel formatting, unknown-event surfaces-as-row, known-ignored
no-spurious-row. Preview "session" frame exercises the clock.

Deferred: the full Go↔Kotlin per-event-type render matrix (§2 last bullet) — a
larger fixture effort than the core behaviors landed here.

apps/tui-go/internal/app/view.go

@@ -13,6 +13,10 @@ const (
 	statusH = 1
 	footerH = 1
 	inputH  = 4 // rounded box: 2 borders + 2 content lines
+
+	// staleEventThresholdMs: past this gap since the last event, an *active* session's
+	// last-event clock turns warn-colored — the "thinking vs hung" tell (§2).
+	staleEventThresholdMs = 30_000
 )
 
 // View renders the whole frame purely from the model (immediate-mode).
@@ -93,6 +97,17 @@ func (m Model) renderStatus() string {
 	left := strings.Join(parts, sep)
 
 	var right []string
+	// Last-event clock (§2): always visible in-session, so a silent agent is never
+	// mistaken for a healthy one. Updates every frame tick; turns warn-colored when an
+	// active session has gone quiet past the stale threshold.
+	if s := m.session(m.selectedID); s != nil && m.displayState() != StateIdle {
+		gap := nowMillis() - s.LastEventAt
+		clockFg := t.P.Faint
+		if s.Active && gap > staleEventThresholdMs {
+			clockFg = t.P.Warn
+		}
+		right = append(right, span("last event "+agoLabel(gap), clockFg))
+	}
 	if g := m.gaugeText(); g != "" {
 		right = append(right, span(g, t.P.Faint))
 	}
@@ -126,6 +141,26 @@ func (m Model) gaugeText() string {
 	return strings.Join(parts, " ")
 }
 
+// agoLabel renders a millisecond age as a compact "Ns ago" / "Nm ago" / "Nh ago" /
+// "Nd ago". Drives the last-event clock (§2) — the live signal that distinguishes a
+// thinking agent from a hung one.
+func agoLabel(deltaMs int64) string {
+	if deltaMs < 0 {
+		deltaMs = 0
+	}
+	s := deltaMs / 1000
+	switch {
+	case s < 60:
+		return itoa(int(s)) + "s ago"
+	case s < 3600:
+		return itoa(int(s/60)) + "m ago"
+	case s < 86400:
+		return itoa(int(s/3600)) + "h ago"
+	default:
+		return itoa(int(s/86400)) + "d ago"
+	}
+}
+
 var spinnerFrames = []string{"⠋", "⠙", "⠹", "⠸", "⠼", "⠴", "⠦", "⠧", "⠇", "⠏"}
 
 func (m Model) spinner() string { return spinnerFrames[m.frame%len(spinnerFrames)] }