Name: xPower LAP & SESSION
Author: MattinG

xPowerLap — Dr. Skiba xPower per Lap and Session

This data field implements Dr. Philip Skiba's xPower (the physiological counterpart to Coggan's Normalized Power). xPower is the 4th-root of the mean of the 4th powers of an exponentially weighted moving average (EWMA) of power with a 25 s time constant. It weights high-power efforts exponentially to reflect metabolic cost (Skiba, 2006-2008).

Lap handling:
xPowerLap maintains two independent states. Total (session) accumulates from activity start and is never reset. Lap state (EWMA_lap, EWMA_4_lap, seg_lap, xPower_lap) is reset on every lap (onTimerLap, manual or AutoLap). The frozen xPower_lap of the finished lap is published to the FIT LAP field before the reset, so the Laps table always gets the correct value. This gives a true per-interval xPower isolated to the current lap, while still showing the whole-ride xPower for comparison.

FIT file (when "Save metrics to .fit file?" is enabled):
 - id0 RECORD FLOAT  "xPower"        — xPower_total sampled every second → continuous chart in Garmin Connect
 - id1 SESSION FLOAT "xPowerSession" — final xPower_total → Activity Summary
 - id2 LAP FLOAT     "xPowerLap"     — xPower_lap frozen per lap → Laps table

Requirements:
 - Power meter required. Recording interval 1 s recommended (works with any interval, dt-exact).
 - Autopause can stay enabled (timerTime based dt).

References:
 - Skiba PF. Calculation of Power Output at Different Effort Durations. 2006.
 - Skiba PF, Clarke D, Vanhatalo A, Jones AM. Validation of a Novel Intermittent W′ Model... 2014 (EWMA/xPower basis). 
 - Coggan AW. Normalized Power concept (xPower is Skiba's exponential variant).