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/ui_app.hpp

@@ -1885,8 +1885,9 @@ gboolean e1608_probe_reconnect(gpointer user);
 gboolean e1608_probe_resumed(gpointer user);
 /// The flat UDP (MARTe2 UDPSender) source: the signal-list codec and a hand-computed decode, then,
 /// against `tools/mock_flatudp.py`, the stream, the value continuity, the wrong-size accounting and
-/// the capture button (`_report`, a second later). `_gap` reads the source inside the mock's silence;
-/// `_resumed` shows the stream came back.
+/// the capture button (`_report`, a second later). `_gap` starts a 100 ms poll that waits for the
+/// stream, then for the mock's silence to drop the source (whenever it falls), then (`_resumed`) for
+/// the stream to come back.
 gboolean flatudp_probe(gpointer user);
 gboolean flatudp_probe_report(gpointer user);
 gboolean flatudp_probe_gap(gpointer user);