* Fix position exchange throttling race condition
Separate tracking of position broadcasts vs replies to fix exchange position functionality.
Previously, allocReply() would refuse to send position replies if any position packet
(broadcast or reply) was sent within the last 3 minutes. This caused the exchange
position feature to fail when a device had recently sent a position broadcast.
Changes:
- Add lastSentReply member to track position reply timestamps separately
- Update allocReply() to only throttle based on previous replies, not broadcasts
- This allows position exchange to work even after recent position broadcasts
The fix maintains the 3-minute throttling for replies to prevent spam while allowing
legitimate position exchange functionality to work properly.
* Remove unused lastSentToMesh variable
Variable was no longer used after separating reply throttling logic.
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