Fix launch detection and cancellation

[PurpleSentinel]

Summary

• replace the impossible all-axis launch threshold with a calibrated acceleration-vector delta
• remove gravity and mounting orientation through a 0.4-second stationary baseline
• filter and debounce the signal to reject isolated vibration while handling either acceleration direction
• add double-tap cancellation and a 30-second safe timeout
• document sensitivity values in g and their practical meaning

Root cause

The previous loop required all three signed acceleration axes to exceed the configured threshold simultaneously. A real vehicle launch normally affects one principal axis while gravity and the other axes can remain below zero, so Launch Mode could wait forever and offered no escape path.

Validation

• python3 -m unittest discover -s tests -v — 41 tests passed
• deterministic stationary, forward-launch, reverse-axis, vibration, cancellation, and timeout cases passed
• connected QMI8658 stationary sampling measured approximately 0.003 g peak-to-peak noise
• MicroPython hardware tests confirmed positive and negative launch detection, ±2 g vibration rejection, real stationary timeout, and cancellation
• connected CST816T integration confirmed the double-tap callback exits and clears the gesture

Fixes #3