j142 retire the ADE+SBC gun (rack id 17): the owner watched it, it does not learn, throw it away

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.

common_libs/tests/test_lead_gain_registration.nim

@@ -37,7 +37,7 @@ proc testTable() =
     if DefaultRackMembership[i] != want: onlyPattern = false
   check "rack: the shipped default is still the onlyPattern rack", onlyPattern
   check "rack: the default rack admits only Pattern (1v1)",
-        admittedGuns(18, rm1v1, DefaultRackMembership) == @[PatternId]
+        admittedGuns(NumRackGuns, rm1v1, DefaultRackMembership) == @[PatternId]
   check "gate: LEADGAIN is NOT spawned under the default rack",
         not vBulletAdmitted(LeadGainId, rm1v1, DefaultRackMembership, true)
 
@@ -49,7 +49,7 @@ proc testEnvOverride() =
         m[LeadGainId] == rmBoth and
         vBulletAdmitted(LeadGainId, rm1v1, m, true)
   check "env: admitting LEADGAIN leaves Pattern as the only other member",
-        admittedGuns(18, rm1v1, m) == @[PatternId, LeadGainId]
+        admittedGuns(NumRackGuns, rm1v1, m) == @[PatternId, LeadGainId]
   clearRackEnv()
 
 proc testLazyAndRngClean() =