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>

CLAUDE.md

@@ -792,6 +792,19 @@ far the largest file.
   `crosshair_interpolated_mode_blends_the_two_samples` and
   `the_crosshair_mark_is_bigger_than_a_sample_dot`, plus `[cursors-probe]`'s `first mark at x=..
   y=..` agreeing with the pane's first value.
+- **`format_measure()` is the one formatter for both panes, and its SI prefix does not depend on
+  there being a unit.** The cursors pane's values and the measurements pane's `text=` go through
+  it, so a change reaches both. The prefix ladder used to be inside `if (!unit.empty())`, which
+  made two decimals of a small unitless value read `0.00` — the reading gone, not merely coarse —
+  and that is a whole class of channels, not a corner: `measure_unit()` returns `""` for any signal
+  whose CONFIG declares no unit, and a **math channel never has one**. The regression net could not
+  see it because `tools/mock_udpstreamer.py` declared `unit="V"` for every data signal; it now takes
+  **`--unit ''`**, and that is the flag to run the measures and cursors probes under when touching
+  this (`text=[636.62m]`, `● sine=585.24m`). With no unit the prefix is written **tight** against
+  the number — `1.34m`, not `1.34 m` — because the space exists to separate the number from its
+  unit and a spaced `m` alone reads as metres; with one it stays spaced and the prefix joins the
+  unit (`636.62 mV`). Below `1e-12` the ladder runs out, and `%.2f` would print `0.00` again, so
+  that range falls back to `%.3g`.
 - **The cursors are read back from the plots only while a gesture is on that row.** `Cursor::x`
   is the *free* pointer position while a drag runs and the snapped value otherwise -- which is
   what the app itself wrote a frame earlier, or the snap of it. A tick that reads a row on