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.

docs/gun_campaign.md

@@ -1153,3 +1153,18 @@ round-boundary bug and its fix, all 4 forced arms' applied shares (exactly
 per-opponent delta, CI, MDE, sign and sign-flip test, and the policy-isolation
 comparison. **INFERRED:** that no share schedule whatsoever could ever help
 (this batch bounds only the shares it tested, at this MDE and panel).
+
+---
+
+## Cross-reference: the ADE+SBC gun (rack id 17) is RETIRED
+
+The `bitbrain` arm named in earlier batches of this ledger is NOT the gun
+shipped at id 16. The **ADE+SBC gun** that briefly held the `BITBRAIN` rack name
+and the `TR_BITBRAIN_*` env prefix at **rack id 17** was **retired and removed
+from the tree** (owner: *"not learning, i watched it, throw it away and we
+forget about it"*). Its tombstone, evidence and removal list are in
+`docs/bitbrain_campaign.md` §RETIRED. The `bitbrain` verdict in this ledger —
+nothing beat Pattern — is unchanged; this line only records that the id-17
+experiment that followed it is closed, so the two ledgers do not contradict
+each other. The surviving "BitBrain-named" capability is the id-16 lead-gain
+corrector, now called **LEADGAIN**.