refactor: share the shade step maths in ShadeMotion.cpp

processFrame worked out a StepUp/StepDown target with the same formula in
eight places, each with its own divide-by-zero guard.  4dd4a98 fixed one
that guarded on downTime but divided by upTime.  stepPosition() takes the
travel time once, for both the guard and the maths, so they can't drift
apart again, and it is now covered by native tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ShadeMotion.h

@@ -15,6 +15,11 @@ float travelPosition(float startPos, int8_t dir, uint32_t elapsedMs, int32_t tra
 int8_t displayPosition(float pos, bool flip);
 // Where a shade (or its tilt) moving in dir ends up when nothing stops it.
 float limitTarget(int8_t dir);
+// Where a step command moves a shade (or its tilt) from pos, in out.  dir > 0
+// steps toward 100 and dir < 0 toward 0.  A step is stepSize * frameStepSize
+// ms of the travelMs a full travel takes in that direction.  False when
+// travelMs or stepSize is 0, so the step is ignored.
+bool stepPosition(float pos, int8_t dir, uint32_t travelMs, uint16_t stepSize, uint8_t frameStepSize, float &out);
 
 // What a held shade does on this pass of checkMovement().
 enum class TxHoldCheck {