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.cpp

@@ -36,6 +36,47 @@ bool stepPosition(float pos, int8_t dir, uint32_t travelMs, uint16_t stepSize, u
   out = dir > 0 ? std::min(100.0f, pos + step) : std::max(0.0f, pos - step);
   return true;
 }
+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) {
+  StepTargets t;
+  if(stepSize == 0) return t;
+  auto atEnd = [dir](float p) { return dir > 0 ? p >= 100.0f : p <= 0.0f; };
+  const uint32_t liftTime = dir > 0 ? downTime : upTime;
+  float step;
+  if(tilt == tilt_types::integrated) {
+    if(atEnd(tiltPos) && atEnd(pos)) return t;
+    if(!atEnd(tiltPos)) {
+      if(!stepPosition(tiltPos, dir, tiltTime, stepSize, frameStepSize, step)) return t;
+      t.setLift = true;
+      t.lift = pos;
+      t.setTilt = true;
+      t.tilt = step;
+    }
+    else {
+      if(!stepPosition(pos, dir, liftTime, stepSize, frameStepSize, step)) return t;
+      t.setLift = true;
+      t.lift = step;
+      t.setTilt = true;
+      t.tilt = tiltPos;
+    }
+  }
+  else if(tilt == tilt_types::tiltonly) {
+    if(!stepPosition(tiltPos, dir, tiltTime, stepSize, frameStepSize, step)) return t;
+    t.setTilt = true;
+    t.tilt = step;
+  }
+  else {
+    if(atEnd(pos)) {
+      t.action = StepAction::none;
+      return t;
+    }
+    if(!stepPosition(pos, dir, liftTime, stepSize, frameStepSize, step)) return t;
+    t.setLift = true;
+    t.lift = step;
+  }
+  t.action = StepAction::move;
+  return t;
+}
 void TxHold::mark(uint32_t ticket, bool moving, uint32_t now) {
   if(ticket == 0 || this->ticket != 0 || moving) return;
   this->ticket = ticket;