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/model.go

@@ -49,6 +49,7 @@ const (
 	OverlayModels
 	OverlayArtifacts
 	OverlayConfig
+	OverlayStats
 )
 
 // RouterEntry is one line in a session's conversation transcript.
@@ -143,15 +144,15 @@ type Model struct {
 	reconnecting bool
 
 	// sessions
-	sessions   []Session
-	selectedID string
-	filter     string
-	workflows  []Workflow
-	wfIndex    int // -1 = not in workflow picker
-	wfVisible  bool
+	sessions      []Session
+	selectedID    string
+	filter        string
+	workflows     []Workflow
+	wfIndex       int // -1 = not in workflow picker
+	wfVisible     bool
 	wfPendingID   string // workflow chosen, awaiting an intent line before StartSession
 	wfPendingName string
-	bgUpdates  int
+	bgUpdates     int
 
 	// input
 	editMode     EditMode
@@ -209,6 +210,11 @@ type Model struct {
 	configRestart []string // keys from the last save that need a restart
 	configLoading bool
 
+	// session stats (OverlayStats) — populated by the session.stats reply
+	stats        *protocol.StatsDto
+	statsFor     string // sessionId the current stats belong to
+	statsLoading bool
+
 	// command palette
 	paletteFilter string
 	paletteIndex  int