flatudp: vector rows as channels or as a waveform, a re-openable source, probes for both

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/source/flatudp/flatudp_source.hpp

@@ -40,7 +40,7 @@ struct FlatUdpSettings {
   std::string problem() const;
   /// `[multicast:]port`; names the session file.
   std::string identity() const;
-  /// Datagrams per frame: 1 for a list with a vector, else `clamp(round(rate / 100), 1, 1024)`.
+  /// Datagrams per frame: 1 for a list with a waveform row, else `clamp(round(rate / 100), 1, 1024)`.
   std::size_t block() const;
 };
 
@@ -78,6 +78,7 @@ class FlatUdpSource final : public DataSource {
   int fd_ = -1;
   bool configured_ = false;
   bool ever_connected_ = false;
+  bool recv_error_noted_ = false;  ///< a failing `recv` is logged once per open
   std::size_t expected_ = 0;  ///< bytes of a valid datagram
   std::size_t block_ = 1;