Motor speed varies with drum radius as shade material winds/unwinds,
causing linear time-interpolation to misestimate position. Adds two
optional calibration fields (upMidTime, downMidTime) that let users
provide the elapsed time at 50% travel. A logarithmic model (alpha
ratio solved via Newton's method) then replaces the linear formula.
Falls back to existing linear behaviour when mid-time fields are zero.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>