feat(navigator): add NavigatorModeChange uORB message for mode transition telemetry

[Rpirayesh]

Summary

During post-flight analysis of autonomous missions, determining why a vehicle transitioned out of a mission or into failsafe currently requires correlating multiple log topics by hand. This PR introduces a dedicated, structured record for every navigator mode transition.

What this adds

A new navigator_mode_change uORB topic published once per mode transition, automatically recorded to ULog. Each message captures:

• nav_state_prev / nav_state_new — the state transition
• nav_state_user_intention — what the user intended (distinguishes commanded vs automatic transitions)
• in_failsafe — whether the vehicle was in failsafe at the time
• triplet_reset_applied — whether reset_triplets() was called on entry
• entry_velocity_xy — horizontal ground speed at the moment of transition [m/s]
• entry_dist_to_target — distance to the active waypoint at the moment of transition [m], NaN if none

Files changed

File	Change
msg/NavigatorModeChange.msg	New message definition
msg/CMakeLists.txt	Register new message in build
src/modules/navigator/navigator.h	Add publisher + publish_mode_change() declaration
src/modules/navigator/navigator_main.cpp	Detect transitions, capture context, publish
src/modules/logger/logged_topics.cpp	Register topic for automatic ULog recording

Why uORB instead of PX4_DEBUG

Debug log strings are stripped in release builds and cannot be queried after a flight. A uORB message is recorded at full fidelity in every ULog, parseable by Flight Review and pyulog, and adds negligible overhead — one small struct publish per mode change, which is a rare event.

Testing

Verified with listener navigator_mode_change in SITL (gazebo-classic iris). Topic publishes correctly on each mode transition. Existing navigator unit tests pass.

make tests TESTFILTER=navigator