COMMAND_INT: frame is ignored for altitude — DO_SET_ROI_LOCATION always treats z as AMSL

[DonLakeFlyer]

Describe the bug

MavlinkReceiver::handle_message_command_int() uses the frame field of COMMAND_INT only to select x/y integer scaling (1e4 for local frames vs 1e7 for global), then discards it. The altitude is copied verbatim (vcmd.param7 = cmd_mavlink.z) with no frame-based conversion, and the internal vehicle_command_s has no frame field, so downstream consumers cannot recover it.

For MAV_CMD_DO_SET_ROI_LOCATION, Commander/navigator then treat param7 as AMSL unconditionally. A command sent in MAV_FRAME_GLOBAL_RELATIVE_ALT is silently misinterpreted: the relative altitude is applied as absolute. PX4 neither converts nor NACKs the unsupported frame.

Per the MAVLink COMMAND_INT spec, frame applies to x/y/z, so this is a spec non-compliance.

To Reproduce

1. Send COMMAND_INT with MAV_CMD_DO_SET_ROI_LOCATION, frame = MAV_FRAME_GLOBAL_RELATIVE_ALT, z = 50 (intending 50 m above home).
2. Command is ACCEPTED; gimbal aims at 50 m AMSL instead of home+50 m.

Expected behavior

Either convert relative/terrain altitudes to AMSL at ingest (plumbing the frame through vehicle_command_s or converting in mavlink_receiver.cpp), or reject unsupported frames with MAV_RESULT_DENIED instead of silently misreading the altitude.

Additional context

• Same bug class as DO_REPOSITION ignores frame, always assumes AMSL #10246 (DO_REPOSITION ignores frame, always assumes AMSL), which was closed stale/not-planned, and Add support for "frame" argument in "COMMAND_INT" version of MAV_CMD_DO_REPOSITION #8502, which fixed only DO_REPOSITION rather than the shared COMMAND_INT ingest path.
• QGroundControl currently works around this for click-to-ROI by running its own terrain query and pre-converting to AMSL before sending in MAV_FRAME_GLOBAL (TerrainQueryCoordinator::roiWithTerrain).
• Code: src/modules/mavlink/mavlink_receiver.cpp, handle_message_command_int() — frame used only for x/y scaling, vcmd.param7 = cmd_mavlink.z unconverted. Verified on v1.18.0-beta2.