AC airborne detection rarely triggers and doesn't hold

[Mhytee]

Issue

Airborne ducking doesn't work in AC: the current detector (all four
wheelLoad[] below 1.0 N, checked per frame, AcSharedMemoryTelemetrySource.cs)
rarely sets the airborne flag and doesn't hold it through a flight.

Test

In AC, set airborne ducking to duck ALL effects at full reduction, so there
should be no haptics at all while airborne. Jump, and also watch the
airborne/grounded edge lines in the SimHub log.

• Before fix: haptics continue or flicker back mid-air; the log shows no
  airborne window or millisecond blips per jump.
• After fix: all haptics cut from takeoff to touchdown and return on landing;
  the log shows one airborne window covering the whole flight. Normal driving
  (kerbs, bumps, standing start) shows no airborne windows.

Fix

Replace the wheelLoad threshold with frozen-value detection: AC stops
updating a wheel's wheelSlip[] entry while that wheel is off the ground, so
all four entries bit-identical to the previous physics frame = airborne.
Proven in Content Manager (gro-ove/actools SessionStats.cs), which also
distinguishes wheelies/two-wheel states from partial freezes. Gate on speed:
a parked car's slip values are also frozen.

Keep the airborne/grounded edge logging so window lengths are verifiable.

Related: #30, #32.