Remove guns/bitbrain_net.nim (+README), test_bitbrain_net.nim,
measure_bitbrain_scaling.nim, rack id 17 and all of its plumbing in
selector.nim / ModularBot.nim / env_report.nim, the TR_BITBRAIN_NET switch
and the NEW-NETWORK TR_BITBRAIN_* knobs, and the BitBrainNet arm of
run_prediction_quality.nim.
With id 17 gone there is nothing to disambiguate, so the legacy namespace
becomes the ONLY one: TR_RACK_BITBRAIN always selects id 16 LEADGAIN and
every TR_BITBRAIN_<X> in the frozen 14-suffix alias set always means
TR_LEADGAIN_<X>. The alias layer and its [depr] line stay.
KEPT: the common_libs/bitbrain/ SBC library (learned_surfer imports
bitbrain/sbc), lead_gain at id 16 with env TR_LEADGAIN_* and log tag [lg],
and the c9b6753 crash fix (NumRackGuns widths + test_rack_stat_width).
Tombstone: docs/bitbrain_campaign.md ## RETIRED and one cross-reference line
in docs/gun_campaign.md. Shipped defaults unchanged: clean env -> rack
active 1v1 = PATTERN, movement default strafe.
SirStone