server: on BlockOnce timeout, fall back to non-blocking Read

Frontends rely on getting the current state on timeout (heartbeat).
When BlockOnce returns empty (5s timeout), the server now calls the
plain Read handler as fallback. Files without a Read handler (like
feed) return nothing — consumer unaffected.

cmd/olliesrv/server.go

@@ -500,6 +500,10 @@ func read(root *fs.Tree, cs *connState, fc *plan9.Fcall, ctx context.Context) *p
 		if err != nil {
 			return errFcall(fc, err.Error())
 		}
+		if len(content) == 0 {
+			// Timeout: fall back to non-blocking Read for current value.
+			content, _ = entry.Read()
+		}
 		if nextBase != "" {
 			cs.mu.Lock()
 			if f, ok := cs.fids[fc.Fid]; ok {