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/server.go

@@ -304,6 +304,10 @@ func (m *Model) applyServer(msg protocol.ServerMessage) {
 		if m.artifactsIndex >= len(m.artifacts) {
 			m.artifactsIndex = 0
 		}
+	case protocol.TypeSessionStats:
+		m.stats = msg.Stats
+		m.statsFor = msg.SessionID
+		m.statsLoading = false
 	case protocol.TypeConfigSnapshot:
 		m.configFields = msg.ConfigFields
 		m.configRestart = msg.ConfigRestartRequired
@@ -332,7 +336,7 @@ func sessionIDOf(msg protocol.ServerMessage) string {
 		protocol.TypeProtocolError, protocol.TypeProviderStatus,
 		protocol.TypeWorkflowList, protocol.TypeRouterResponse,
 		protocol.TypeModelChanged, protocol.TypeModelList, protocol.TypeResourceStatus,
-		protocol.TypeArtifactList, protocol.TypeConfigSnapshot:
+		protocol.TypeArtifactList, protocol.TypeConfigSnapshot, protocol.TypeSessionStats:
 		return ""
 	default:
 		return msg.SessionID