Most Demanding Phase - Horizontal Acceleration

**CLI Output Name**: -o mostAccelPhase

**Input**: a flight record file

**Print**: the name of the phase with the highest average horizontal acceleration: phase_name:acceleration (m/s^2)

Start by locating the CLI entry point that handles the -o output name and the code that reads a flight record file. Trace how flight phases and acceleration values are represented, then run the existing CLI checks if available. Done means -o mostAccelPhase prints the phase name and its highest average horizontal acceleration in the form phase_name:acceleration (m/s^2).

Written by the indexing model from the issue text.