refactor: pin which radio job holds a shade's simulated movement

Review flagged the outer My/Favorite losing to the nested Up/Down it queues.
Every such nested send is from a simMy shade, where the Up/Down is what
starts the motor, so the behaviour stays.  The hold moves into TxHold in
ShadeMotion with a comment explaining the order and unit tests pinning it.

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

src/ShadeMotion.h

@@ -12,4 +12,23 @@ float travelPosition(float startPos, int8_t dir, uint32_t elapsedMs, int32_t tra
 // A position as reported to clients: -1 when unset, otherwise the whole
 // percentage, counted from the other end when flipped.
 int8_t displayPosition(float pos, bool flip);
+
+// The radio job that set an idle shade moving.  Its simulated movement is held
+// until the job goes on air, since that is when the motor starts.
+struct TxHold {
+  uint32_t seq = 0;    // 0 when nothing is held.
+  uint32_t since = 0;  // millis() when the hold started.
+  // Holds for job seq unless the shade was already moving (its motor carries on
+  // with the earlier command) or a job is already held.  The first job held is
+  // the one that starts the motor.  A command sent while processing another
+  // registers before it: for a simMy shade My/Favorite queues its Up/Down from
+  // moveToMyPosition() first, and that Up/Down is what starts a motor with no
+  // favorite of its own, so the outer My/Favorite is ignored on purpose.
+  void mark(uint32_t jobSeq, bool moving, uint32_t now);
+  // Ends the hold when jobSeq is the held job.  True when it was.
+  bool release(uint32_t jobSeq);
+  bool held() const { return this->seq != 0; }
+  bool timedOut(uint32_t now, uint32_t timeoutMs) const { return now - this->since >= timeoutMs; }
+  void clear() { this->seq = 0; }
+};
 #endif