Autopilot: rescue as a synthesised mission, TAKEOFF and hold patterns, SITL fidelity - #15413

…CUE_PLAN

With ENABLE_RESCUE_PLAN (default off, byte-identical builds otherwise) the
failsafe GPS-RESCUE procedure stages a 3-waypoint mission — altitude-gated
climb-in-place HOLD, FLYOVER home, LAND — that flightPlanNavEngage consumes
in place of the PG plan; core.c engages the executor via the existing
FAILSAFE_AUTOPILOT phase with a 1 s grace window, and a mission that cannot
start or aborts degrades to the baro-only auto-landing rather than
re-staging itself. Return altitude mirrors legacy initRescueValues per
altitude mode (a behaviour-neutral getter exposes the tracked maximum) and
never commands an en-route descent. While the IMU heading is untrusted at
the top of the climb the executor holds the plan and pitches forward until
GPS course-over-ground teaches the estimator its heading, with the legacy
15 s give-up aborting to the failsafe fallback. The BOXGPSRESCUE switch
keeps flying legacy rescue and preempts the mission.

Engage now captures the estimator-vs-GPS altitude frame offset instead of
the raw estimator reading, which shifted every waypoint up by the current
height when a rescue engaged mid-flight.

SITL A/B harness: --binary-b runs rescue scenarios against both
implementations and compares qualitative outcome metrics; scenarios cover
the plain return, heading recovery from a 90 degree error without a mag,
and GPS loss mid-return. Blackbox .BFL artifacts are captured per leg.

TAKEOFF waypoints climb at the current position to the waypoint altitude
(lat/lon advisory, matching MAV_CMD_NAV_TAKEOFF), gated on altitude
arrival, with duration as a post-climb loiter.

HOLD waypoints with a pattern chase a time-parametrised carrot around
the hold point at ap_waypoint_hold_radius. The carrot command never
completes (completionSpeed 0) so the hold timer and callback are
undisturbed, and positionNavMoveTargetEf() moves the target each step
without resetting the velocity ramp. Pattern start is deferred until
the arrival braking settles, and the carrot rate is capped at 0.25
rad/s; entry momentum or faster rotation than the control chain can
phase-track balloons the pursuit orbit well outside the ring.

WAYPOINT_PATTERN_NONE = 0 keeps stored missions station-keeping.
MAVLink LOITER_TURNS now maps to an ORBIT hold, with the turn count
converted to and from a hold duration at the executor's orbit rate.