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>

src/help/09-cursors.txt

@@ -22,6 +22,12 @@ owns a <b>Reset</b> button. Drag a marker to move it. The mode dropdown offers f
 • A value is the <i>nearest stored sample's</i>, never a number between two samples. Zoom in past
   the sample spacing and the line between two samples is something the plot drew, not data —
   so a reading off it would be the drawing. The hover crosshair reads the same way.
+• Every reading carries a <i>metric prefix</i> so the digits sit in <tt>1..999</tt>:
+  <tt>10.52 mV</tt>, not <tt>0.01 V</tt>. Two decimals is all an instrument landing on an
+  <tt>f32</tt> sample can claim, so without the prefix a small value would read <tt>0.00</tt>
+  and the reading would be gone. A stream that declares no unit gets the prefix anyway, written
+  tight against the number (<tt>585.24m</tt>) — the space separates the number from its unit,
+  and a spaced <tt>m</tt> with nothing after it would read as metres.
 • A marker past the newest sample reads <tt>--</tt>. With only one marker over data the row
   reads <tt>partial</tt>: a difference between a number and nothing is not a number.
 • In <tt>mix</tt> each marker also places a crossing annotation per trace, which is what lets