The drone sends BatteryStateChanged from two places with different values: the
true figure on buffer 126, corroborated by its own log and tracking a smooth
discharge, and a constant 0 on buffer 127 that appears in no log line. Keeping
the newest frame meant a healthy aircraft read as flat and preflight refused.
Events now carry their buffer and the store keeps the last value per source, so
a disagreement is visible instead of silently resolved. get_state reports the
sources whenever they differ, rather than handing over a winner.
preflight breaks the tie on physical grounds, not preference: an aircraft that
is powered and holding a link is not at 0%, so a zero from a live drone is not
credible while a non-zero one is. It says the number was contested either way,
and a genuine zero from every source still reports zero and still blocks.
