fix(commander): convert the baro timestamp delta to seconds for the home altitude filter (#28415)

The low pass filter smoothing barometric altitude for the in-air home
position correction was fed the raw timestamp difference. uORB timestamps
are microseconds, but AlphaFilter::setParameters() documents both of its
arguments as seconds and the filter is constructed with a 5 second time
constant, so the sample interval arrived a million times too large:

  alpha = dt / (tau + dt)

  rate     dt        alpha (before)   alpha (intended)
  50 Hz    0.0200 s        0.999750           0.003984
  100 Hz   0.0100 s        0.999500           0.001996
  200 Hz   0.0050 s        0.999001           0.000999

At an alpha of 0.9997 the filter passes essentially every raw sample
through, giving an effective time constant of 5 us instead of 5 s, so
_lpf_baro.getState() has been effectively unfiltered barometric altitude.

That state feeds the in-air home altitude correction: it is offset by
_baro_gps_static_offset and then compared against the GNSS altitude, and
home.alt is shifted when the two differ by more than
kAltitudeDifferenceThreshold. A GNSS velocity integral gates that
comparison for consistency.

The same conversion is already done correctly for the GNSS integral a few
lines below in this file, and for the geoid height filter in EKF2.

Note that this does change behaviour: the filter now actually applies its
5 s time constant, so _lpf_baro.getState() lags during a climb by roughly
the time constant times the climb rate. _baro_gps_static_offset is
captured once when the correction window opens, so that lag does not
cancel and it biases baro_alt_corrected while climbing. Reviewers who
know this feature should say whether the 5 s constant and the 1 m
threshold, both tuned while the filter was effectively a pass-through,
still want the same values now that it filters.

Assisted-by: Claude:claude-fable-5

Signed-off-by: Saibernard Yogendran <bernie97@seas.upenn.edu>

src/modules/commander/HomePosition.cpp

@@ -364,7 +364,7 @@ void HomePosition::update(bool set_automatically, bool check_if_changed)
 		const float baro_alt = baro_data.baro_alt_meter;
 
 		if (_last_baro_timestamp != 0) {
-			const float dt = baro_data.timestamp - _last_baro_timestamp;
+			const float dt = 1e-6f * (baro_data.timestamp - _last_baro_timestamp);
 			_lpf_baro.update(baro_alt, dt);
 
 		} else {