feat(commander): uncommanded altitude loss detection with parachute failsafe

Detects when a rotary-wing vehicle drops more than FD_ALT_LOSS metres
below a NED-z reference while altitude control is active,
and immediately triggers flight termination (parachute deployment).

Detection (FailureDetector):
- FD_ALT_LOSS: drop threshold in metres (0 = disabled, default)
- FD_ALT_LOSS_T: hysteresis time
- Guards: rotary-wing only, altitude control active, z_valid, setpoint
  fresh (<1 s). Manual, Acro and FW/VTOL-FW modes are excluded.
- Ratcheting reference: initialises to lpos.z on first sample below
  setpoint, preventing false triggers on new waypoints

Failsafe action (commander):
- New fd_alt_loss flag in FailsafeFlags.msg
- COM_ALT_LOSS_ACT: -1=Disabled (default), 0=Terminate
- Terminate fires immediately, cannot be overridden, and never clears
  until disarm (parachute deployment is irreversible)

msg/FailsafeFlags.msg

@@ -45,10 +45,11 @@ bool battery_low_remaining_time       # Low battery based on remaining flight ti
 bool battery_unhealthy                # Battery unhealthy
 
 # Failure detector
-bool fd_critical_failure              # Critical failure (attitude/altitude limit exceeded, or external ATS)
+bool fd_critical_failure              # Critical failure (attitude limit exceeded, or external ATS)
 bool fd_esc_arming_failure            # ESC failed to arm
 bool fd_imbalanced_prop               # Imbalanced propeller detected
 bool fd_motor_failure                 # Motor failure
+bool fd_alt_loss                      # Uncommanded altitude loss (rotary-wing, altitude-controlled flight)
 
 # Other
 bool geofence_breached        	      # Geofence breached (one or multiple)