fix: a readout keeps its metric prefix when the stream declares no unit

The SI prefix ladder in format_measure() was gated on a non-empty unit, so two
decimals of a small unitless value printed `0.00` -- the reading gone, not merely
coarse. That is a whole class of channels rather than a corner: a CONFIG need not
declare a unit, and a math channel never has one. Below 1e-12 the same loss
happened even with a unit, so that range falls back to three significant digits.

With no unit the prefix goes tight against the number (`585.24m`), since the space
exists to separate the number from its unit and a spaced `m` alone reads as metres.

The mock declared `unit="V"` for every data signal, which is why no probe could see
this; it now takes `--unit ''`.

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

tools/mock_udpstreamer.py

@@ -50,16 +50,16 @@ def signal_desc(name, *, rate, time_mode, time_idx, unit, rows=1, cols=1, dims=0
     return bytes(d)
 
 
-def build_config(rate, time_mode=TIME_FULL_ARRAY):
+def build_config(rate, time_mode=TIME_FULL_ARRAY, unit="V"):
     sigs = [
         # The stream's clock: referenced by every data signal, so udpscope treats
         # it as the time base rather than a plottable channel. Its *own* time_mode
         # is what udpscope switches on, so `--time-mode first` has to land here.
         signal_desc("time", rate=0.0, time_mode=time_mode,
                     time_idx=NO_TIME_SIGNAL, unit="s"),
-        signal_desc("sine", rate=rate, time_mode=time_mode, time_idx=0, unit="V"),
-        signal_desc("square", rate=rate, time_mode=time_mode, time_idx=0, unit="V"),
-        signal_desc("ramp", rate=rate, time_mode=time_mode, time_idx=0, unit="V"),
+        signal_desc("sine", rate=rate, time_mode=time_mode, time_idx=0, unit=unit),
+        signal_desc("square", rate=rate, time_mode=time_mode, time_idx=0, unit=unit),
+        signal_desc("ramp", rate=rate, time_mode=time_mode, time_idx=0, unit=unit),
     ]
     payload = struct.pack("<I", len(sigs)) + b"".join(sigs)
     payload += bytes([PUBLISH_ACCUMULATE])
@@ -101,6 +101,10 @@ def main():
                          "sample (UDPS_TIME_FIRST_SAMPLE) and leaves the viewer to space the "
                          "rest by the declared cadence -- the shape a real MARTe2-style "
                          "producer with a single u64 clock has")
+    ap.add_argument("--unit", default="V",
+                    help="the unit the data signals declare in CONFIG; '' declares none, which "
+                         "is an ordinary stream (and what every math channel looks like) and is "
+                         "the only way a probe can see the unitless readout formatting")
     ap.add_argument("--clock-jitter", type=float, default=0.0,
                     help="per-packet stamp jitter, as a fraction of one packet's span: a device "
                          "that stamps each batch from its own clock does not land on a perfect "
@@ -138,7 +142,7 @@ def main():
                     t_stream = 0.0
                     started = time.monotonic()
                     next_send = started
-                    sock.sendto(build_config(declared, time_mode), peer)
+                    sock.sendto(build_config(declared, time_mode, args.unit), peer)
                     print("mock udpstreamer: CONNECT from %s:%d, CONFIG sent" % peer, flush=True)
                 elif magic == MAGIC and ptype == PKT_DISCONNECT:
                     print("mock udpstreamer: DISCONNECT", flush=True)