working on todo.md features

src/help/03-signals.txt

@@ -13,6 +13,9 @@ the overflow menu opens the editor where the unit and the calibration live.
 
 • Signals start disabled, and enabling one is what starts acquiring it. A trigger on a signal
   nobody acquires can never fire, so arming a trigger enables its signal for you.
+• <b>Calibrate</b> in a signal's row menu opens the gain and offset it is <i>currently</i> drawn
+  through, so a trace already scaled by a gain of 2 shows 2 — pressing <i>Apply</i> confirms it
+  rather than resetting it. A signal nobody calibrated shows the identity, 1 and 0.
 • Calibration is applied <i>at ingest</i>, so changing it re-plans the buffer and drops the
   history. So does enabling or disabling a signal, and so does a new CONFIG. An empty plot
   for a second after one of those is the ring refilling, not a fault.