fix(navigator): keep course setpoint NaN outside Course mode

[mrpollo]

Follow-up to #27537 and #27487, fixing the course initialization issue at the source so the hotfix guard can be removed.

The PositionSetpoint.course field could end up finite (and get consumed as a course guidance command) in two ways: the reposition triplet was zero-initialized at boot and memset after consumption while none of the reposition command handlers write course, and mission_item_to_position_setpoint() updates the current setpoint in place without ever touching course, so a stale value set by Course mode survived a switch to Hold, RTL, or Mission.

• mission_item_to_position_setpoint() now clears course to NaN; mission items never command a course.
• Loiter::reposition() resets the consumed reposition triplet with reset_position_setpoint() instead of memset, and the navigator resets it once at startup, so unset fields (yaw, course) are NaN rather than zero. This replaces the consumer-side clear from fix(navigator): clear course on loiter reposition #27537.
• Reverts the GUIDED_COURSE nav state guard from fix(fw_mode_manager): guard course guidance on GUIDED_COURSE nav state #27487: with the navigator guaranteeing NaN semantics, PX4_ISFINITE(course) is again a sufficient trigger in the FW mode manager.

Verified in SIH SITL (airplane) at the uORB level: entering Course mode publishes a finite course, switching back to Hold publishes a LOITER setpoint with course = NaN (previously stale finite), and repeated DO_REPOSITION commands produce loiter setpoints with course/yaw = NaN on both the first-use and post-consumption paths. Built for px4_sitl_sih and px4_fmu-v6x.

Log of the SITL session (the position_setpoint_triplet.current.course transitions are visible in Flight Review; ground roll only, the SIH airplane cannot reach rotation speed): https://logs.px4.io/plot_app?log=d89b1ec6-4cc4-4e92-b115-55a90623052c

Changelog Entry

Bugfix: initialize unused course setpoint to NaN so GCS repositions and mode switches out of Course mode never trigger course guidance

@mahima-yoga this should be the "remaining corner cases" hunt from #27537: the stale-course-after-Course-mode path via mission_item_to_position_setpoint() was the one left, covered here along with the revert of your guard.

[github-actions]

🔎 FLASH Analysis

px4_fmu-v5x [Total VM Diff: 16 byte (0 %)]

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 --------------  -------------- 
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px4_fmu-v6x [Total VM Diff: 16 byte (0 %)]

    FILE SIZE        VM SIZE    
 --------------  -------------- 
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    +0.8%     +32  +0.8%     +32    Navigator::run()
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  +0.0%    +136  +0.0%     +16    TOTAL

Updated: 2026-07-03T20:51:59