fix(pier): reap stateful sessions whose store record expired

- a reaper started at Mount checks every local session against the store
  every min(idle_ttl/2, 1m) and closes each copy whose record is gone,
  logging the expired step (A-CR-01, A-IN-06)
- the GET stream re-check ends the stream when the record is gone (A-IN-01)
- tool calls served without a session on the stateful path log the
  session as ephemeral (A-IN-06)
- TestPierReaperSmoke and the ReapNow, LocalSessions, StopReaper seams

modules/pier/audit.go

@@ -22,11 +22,15 @@ func logger() *slog.Logger {
 }
 
 // sessionPrefix shortens a session id for logging; a stateless call logs
-// "stateless".
+// "stateless" and a call served without a session on the stateful path
+// "ephemeral".
 func sessionPrefix(id string) string {
 	if id == "" {
 		return stateless
 	}
+	if id == ephemeralSession {
+		return ephemeralSession
+	}
 	if len(id) > sessionPrefixLen {
 		return id[:sessionPrefixLen]
 	}
@@ -63,9 +67,12 @@ func logToolCall(log *slog.Logger, id Identity, t *tool, session string, status
 	)
 }
 
-// logSession writes one line per session lifecycle step (open, rehydrate,
-// end, binding_refused, expired) with the session prefix and the binding,
-// never the token.
+// logSession writes one line per session lifecycle step with the session
+// prefix and the binding, never the token. The steps: open (serveNew),
+// rehydrate (lookup), end (DELETE), binding_refused (another binding
+// presented the id), revoked (a revocation or deletion event),
+// stream_closed (a GET stream whose bearer or record is gone) and expired
+// (the reaper closed a local copy whose record expired).
 func logSession(log *slog.Logger, step, session, binding string) {
 	log.Info("pier: session", "step", step, "session", sessionPrefix(session), "binding", binding)
 }