feat(server,tui): session stats pane — metrics over the WS bus + Bubble Tea overlay

Surfaces the observability metrics (observability-spec §3-tier-2) in the Go TUI:
a session-summary pane showing duration, token throughput per provider, tool
time, approval latency per tier, failure counts, and the session wall-time
accounting split. Press `S` (or palette → "session stats").

Mirrors the artifacts/config fetch pattern exactly — a WS request/response, not a
new transport. Server-side reuses the already-tested MetricsInspectionService
(one MetricsReport definition, two wires: REST `correx stats` + WS).

Server:
- ClientMessage.GetSessionStats + ServerMessage.SessionStats(@SerialName
  session.stats), reusing MetricsReport as the nested payload.
- StreamQueries.sessionStats replays via MetricsInspectionService (pure read,
  replay-neutral); routed in GlobalStreamHandler.
- ServerMessageSerializationTest golden pins the session.stats wire format.

TUI (Go/Bubble Tea):
- protocol: TypeSessionStats + StatsDto/nested structs + GetSessionStats encoder
  + golden decode test (cross-language contract).
- OverlayStats pane (statsModal), `S` key + palette entry + footer hint,
  loading/cache-by-session state, bounded breakdown rows.
- demo `stats` preview case for serverless visual verification.

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

@@ -155,7 +155,7 @@ func (m Model) renderFooter() string {
 	case m.displayState() == StateIdle:
 		hints = []string{hint("i", "name"), hint("/", "filter"), hint("enter", "open"), hint("jk", "move"), hint("w", "workflows"), hint("p", "cmds"), hint("q", "quit")}
 	default: // StateInSession
-		hints = []string{hint("i", "message"), hint("e", "events"), hint("t", "tools"), hint("m", "model"), hint("s", "mode"), hint("l", "back"), hint("p", "cmds"), hint("q", "quit")}
+		hints = []string{hint("i", "message"), hint("e", "events"), hint("t", "tools"), hint("S", "stats"), hint("m", "model"), hint("s", "mode"), hint("l", "back"), hint("p", "cmds"), hint("q", "quit")}
 		if s := m.session(m.selectedID); s != nil && s.Pending != nil {
 			hints = append(hints, hint("a", "approval (pending)"))
 		}