After Phase J, the joystick UI rewrites a sent channel message's
path_len from OUT_PATH_SENT (0xFE) to OUT_PATH_SENT_HEARD (0xFD) or
OUT_PATH_SENT_UNHEARD (0xFC) once the feedback window resolves.
buildChannelReplyPrefix only excluded OUT_PATH_SENT, so a reply to
one of the user's own messages (post-feedback) would treat it as
incoming and build a garbled "@[<random body>]" prefix.
Also cleans up an awkward `class ContentionTracker&` qualifier in
Mesh.h.