refactor: share step targets between remote and group steps

A received step and a group step each worked out where a step takes the
shade, the group copy with its own inline maths. stepTargets() in
ShadeMotion now does it for both: a tilt-only shade steps its tilt, an
integrated tilt turns its slats to the end first and then steps the lift,
and every other shade steps its lift.

Only edge cases change. A group step on a tilt-only shade already at the
end now goes through like a remote step, and a step that can't be sized
(no travel time) leaves the targets alone in both.

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

src/ShadeMotion.h

@@ -2,6 +2,7 @@
 #define SHADE_MOTION_H
 #include <Arduino.h>
 #include "TxBuffer.h"
+#include "ShadeTypes.h"
 
 // Shade position maths.  Kept free of the shade state, radio and network so it
 // can be unit tested on the PC (pio test -e native).
@@ -20,6 +21,25 @@ float limitTarget(int8_t dir);
 // 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 step command does to a shade.
+enum class StepAction : uint8_t {
+  ignore,  // The step can't be sized or the shade is at both ends: drop it.
+  none,    // A lift shade already at the end: nothing moves.
+  move     // Set the targets below.
+};
+struct StepTargets {
+  StepAction action = StepAction::ignore;
+  bool setLift = false;
+  float lift = 0.0f;
+  bool setTilt = false;
+  float tilt = 0.0f;
+};
+// Where a step in dir (> 0 down, < 0 up) takes a shade at pos and tiltPos.  A
+// tilt-only shade steps its tilt.  An integrated tilt turns its slats to the
+// end first, holding the lift, and then steps the lift.  Every other shade
+// steps its lift.
+StepTargets stepTargets(tilt_types tilt, int8_t dir, float pos, float tiltPos,
+    uint32_t upTime, uint32_t downTime, uint32_t tiltTime, uint16_t stepSize, uint8_t frameStepSize);
 
 // What a held shade does on this pass of checkMovement().
 enum class TxHoldCheck {