gig-up runs check-preload --fix whenever the check fails, so a plan that was a
deliberate superset scored STALE and got silently narrowed — bass2 would have
been dropped on gig night and ete_a_mauerpark would have lazy-loaded mid-set.
Membership now uses comm -13/-23 (missing is a fault, extra is information),
counts join on shared banks, and --fix emits the union so it can never shrink.

Second hole, latent: under en_US.UTF-8 a whole-line sort orders 'bass2 5' before
'bass 4' while join compares field 1, so join dropped a pair (123 of 124 shared
banks) and 2>/dev/null ate its warning — a count check with a hole reports ok.
LC_ALL=C and sort -k1,1 on every set operation; join now equals comm -12.
Extras are also count-checked against disk: SuperDirt asserts the recorded count
at boot whether or not the bank is in the set.