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_rack_membership.nim

@@ -158,14 +158,14 @@ proc testDefaultsOnlyPattern() =
         DefaultRackMembership[12] == rmOff and DefaultRackMembership[13] == rmOff and
         DefaultRackMembership[14] == rmOff
   check "defaults: the default rack admits ONLY Pattern in 1v1",
-        admittedGuns(18, rm1v1, DefaultRackMembership) == @[PatternId]
+        admittedGuns(17, rm1v1, DefaultRackMembership) == @[PatternId]
   check "defaults: the default rack admits ONLY Pattern in melee",
-        admittedGuns(18, rmMelee, DefaultRackMembership) == @[PatternId]
+        admittedGuns(17, rmMelee, DefaultRackMembership) == @[PatternId]
   let loaded = loadRackMembership()
   check "defaults: with a clean environment loadRackMembership() == shipped table",
         loaded == DefaultRackMembership
-  check "defaults: RackGunNames covers the shipped 18-gun rack",
-        RackGunNames.len == 18 and DefaultRackMembership.len == 18
+  check "defaults: RackGunNames covers the shipped 17-gun rack",
+        RackGunNames.len == 17 and DefaultRackMembership.len == 17
 
 proc testFloorRespectsAdmission() =
   ## The FLOOR path (`bestRate <= 0` or below the floor rate) historically fell
@@ -204,23 +204,23 @@ proc testRevertOverrideRestoresFullRack() =
   ## stays `off`). This must reproduce the old full rack without a rebuild.
   clearRackEnv()
   putEnv("TR_RACK_PATTERN", "both")
-  # The ADE+SBC gun (id 17) additionally needs TR_BITBRAIN_NET=1; without the
-  # switch TR_RACK_BITBRAIN is the LEGACY name for LEADGAIN (id 16), so the
-  # documented one-liner has to flip the namespace over as well.
-  putEnv("TR_BITBRAIN_NET", "1")
   for name in RackGunNames:
     if name != "TMPATTERN": putEnv("TR_RACK_" & name, "both")
   let m = loadRackMembership()
-  delEnv("TR_BITBRAIN_NET")
   var full = true
   for i in 0..<RackGunNames.len:
     let want = if i == 14: rmOff else: rmBoth
     if m[i] != want: full = false
   check "revert: the documented one-liner restores the all-`both` full rack", full
   check "revert: 1v1 rack admits every gun again (TMPATTERN excluded)",
-        admittedGuns(18, rm1v1, m) == @[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 15, 16, 17]
-  check "revert: the legacy TR_RACK_BITBRAIN alone still selects LEADGAIN only",
-        loadRackMembership()[17] == rmOff
+        admittedGuns(17, rm1v1, m) == @[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 15, 16]
+  clearRackEnv()
+  # The ADE+SBC gun (old rack id 17) is RETIRED, so `TR_RACK_BITBRAIN` is now
+  # unambiguously LEADGAIN's old name: alone, it admits LEADGAIN and nothing else.
+  putEnv("TR_RACK_BITBRAIN", "both")
+  let mLegacy = loadRackMembership()
+  check "revert: the legacy TR_RACK_BITBRAIN alone selects LEADGAIN (16) only",
+        mLegacy[16] == rmBoth and admittedGuns(17, rm1v1, mLegacy) == @[5, 16]
   clearRackEnv()
 
 proc testEnvOverrides() =