feat(tui): event-derived ordering + retire Kotlin TUI

Rebuild the Go TUI transcript from the event stream instead of
optimistic local echoes. ChatTurnEvent now maps to ServerMessage.ChatTurn
(chat.turn); the TUI auto-vivifies sessions and renders user+router turns
from events. SessionStarted is de-special-cased into SessionAnnounced
(still carries workflowId), dropping the router.response shadow and
optimistic echoes.

Delete the unused Kotlin TUI (apps/tui) and add a kotlinx<->Go
golden-fixture test to lock the wire protocol. Gitignore server runtime
logs.

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

@@ -237,6 +237,19 @@ func (m Model) session(id string) *Session {
 	return nil
 }
 
+// ensureSession returns the session with id, creating an ACTIVE entry if absent.
+// Session existence is derived from the event stream (any event for an unknown
+// session vivifies it) rather than a dedicated control frame.
+func (m *Model) ensureSession(id string) *Session {
+	if s := m.session(id); s != nil {
+		return s
+	}
+	m.sessions = append(m.sessions, Session{
+		ID: id, Status: "ACTIVE", LastEventAt: nowMillis(),
+	})
+	return &m.sessions[len(m.sessions)-1]
+}
+
 // filteredSessions applies the workflow-id filter.
 func (m Model) filteredSessions() []Session {
 	if m.filter == "" {