flatudp: the signal-row editor form with a per-row Vector as drop-down, its probe line, README and architecture notes

Multicast was verified on a loopback group (239.255.42.99 on 127.0.0.1); a real network was not exercised.

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

docs/claude/verification.md

@@ -442,8 +442,8 @@ precedent, not an exception.
 - `UDPSCOPE_PROBE_E1608_RECONNECT=8000` -- reads the source after the mock dropped the link and 500 ms
   later checks the frame count moved on. The reading's own entry is in `probe-readings.md`.
 - `UDPSCOPE_PROBE_FLATUDP=2500` with `UDPSCOPE_EXIT_AFTER_MS`, run as `--source flatudp` against
-  `tools/mock_flatudp.py` -- the flat UDP source end to end: a hand-computed codec and decode check (it
-  needs no source; it covers the `w` suffix, the waveform and 256-channel rules, the channel decode and the
+  `tools/mock_flatudp.py` -- the flat UDP source end to end: a hand-computed codec and decode check and the connect form's validity rules and signal-row editor (both
+  need no source; it covers the `w` suffix, the waveform and 256-channel rules, the channel decode and the
   float64/uint16/int32/uint32/int64 types in both byte orders), the stream's channel names (`Wave[0]`..
   for a `channels` vector) and declared rates (`rate * count` for a waveform), the channel count, value
   continuity per row (and the offset between a vector's elements, or a waveform's block phase), the