MAVLink and MSP command-control parity

[xznhj8129]

⚠ Part 6/7 of the mavlink_multiport2 split — depends on #11716.
GitHub can't base an upstream PR on a fork branch, so this targets maintenance-10.x and its diff also contains the earlier parts of the stack. Review only this part's own commits:

• `57cb25ba5` Add MAVLink and MSP command-control parity

This part in isolation: xznhj8129/inav@mav/05-mission...mav/06-command-guided
Merge order: parts 1 → 7 in sequence.

Part 6/7 of the mavlink_multiport2 stack.

Command-control parity between MAVLink, MSP, and the Programming Framework, so GCS and companion software can arm, navigate, and land through standard commands.

Arming

• MAV_CMD_COMPONENT_ARM_DISARM and MSP2_INAV_ARM_DISARM (0x2227) go through the normal INAV arming/disarming path and ACK success only when the requested state is actually reached.

RTH

• MAV_CMD_NAV_RETURN_TO_LAUNCH, the ArduPilot-style MAV_CMD_DO_SET_MODE RTL request, MSP2_INAV_ACTIVATE_RTH (0x2225), and Programming Framework operation 61 all enter the normal INAV RTH mode path via activateRTHMode(), which adds a temporary BOXNAVRTH source to the RC mode selector. This deliberately does not use the failsafe/geozone forced-RTH latch — live testing showed activateForcedRTH() is not equivalent to switching into RTH mode. The temporary source clears on a pilot RC flight-mode change or disarm, so a command-triggered RTH cannot survive into a later arming attempt.

Pause / position hold

• MAV_CMD_DO_SET_MODE also accepts ArduPilot pause-style modes (PLANE_MODE_LOITER, COPTER_MODE_LOITER, COPTER_MODE_POSHOLD, COPTER_MODE_BRAKE) and enters normal INAV PosHold at the current position/altitude/heading via a temporary BOXNAVPOSHOLD source (this is what QGC's Pause button sends). Other GCS mode changes remain UNSUPPORTED.

Landing

• MAV_CMD_NAV_LAND, MSP2_INAV_ACTIVATE_LANDING (0x2224), and PF operation 62 perform a normal waypoint-style landing at the current position through a transient waypoint — the uploaded mission is not modified, and this is not emergency landing. Mission LAND items keep their supplied coordinates; direct LAND commands ignore coordinate fields.

Home, time, guided

• MAV_CMD_DO_SET_HOME uses the existing waypoint-0 backend (param1 = 1 = current GNSS position; explicit global location otherwise) and retains all native WP#0 safety gates. MSP set-home remains MSP_SET_WP waypoint 0 — no duplicate command added.
• MSP2_INAV_TIMESYNC (0x2228) returns the same monotonic nanosecond boot clock as MAVLink TIMESYNC.
• SET_POSITION_TARGET_GLOBAL_INT / _LOCAL_NED guided handling (waypoint 255 / altitude-only requests, gated on valid GCS navigation).
• MAV_CMD_CONDITION_YAW heading changes while the nav state has yaw control.
• Temporary fixed-wing PosHold loiter-radius override: MAV_CMD_DO_REPOSITION.param3 in meters, or int32 loiterRadius (cm) appended to MSP2_INAV_SET_GLOBAL_TARGET (17-byte payload; 13-byte payload = no change; 0 clears). MSP2_INAV_NAV_TARGET reports the active override. Volatile: cleared on disarm/reboot, applied only while the nav FSM is in PosHold so RTH/mission/autoland loiter keeps configured defaults.
• Explicit MAV_CMD_NAV_TAKEOFF stub returning UNSUPPORTED (foundation for later takeoff work, no behavior introduced).
• OSD flight-mode element shows GCSN while GCS-assisted navigation is actually active.

All float command parameters are validated (finite/range) before integer conversion.

Companion PR: mspapi2:mavlink_multiport2 (synchronized message metadata + arm/RTH/land/timesync helpers).

Testing

• Unit slice: 81/81 passing (mavlink_unittest).
• Full SITL build, warnings-as-errors, clean. MSP dev docs regenerated.
• X-Plane + QGC SITl test successful

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