j140 rebuild a real BitBrain gun: ADE+SBC at rack id 17, default off, and measure its scaling

The BITBRAIN name was sitting on a gun with no network in it. This is the gun
that actually runs the algorithm: an ADE layer (thresholded random projections
with ONLINE threshold adaptation) feeding the SBC head from
common_libs/bitbrain/, with the counted+decay mode available.

  common_libs/guns/bitbrain_net.nim     the gun
  rack name BITBRAIN, rack id 17 (rack 17 -> 18 guns), both new guns default OFF
  admitted by TR_RACK_BITBRAIN=both AND TR_BITBRAIN_NET=1 (the switch that also
  disowns LEADGAIN's legacy TR_BITBRAIN_* aliases)

OUTPUT: a fine-grained aim CORRECTION on top of Pattern - the probability-
weighted mean of the nClasses class centres under inferProb - not a direct aim
point from the argmax. That is the shape docs/bitbrain_gate.md measured, and
Pattern is already a strong predictor, so the net's job is the signed residual.
Below TR_BITBRAIN_MINOBS the shift is exactly 0 and Pattern is returned
unchanged.

INPUT: a CONFIGURED set of FEATURE BLOCKS (TR_BITBRAIN_FEATURES=name:W), each
block's width == its resolution, laid out as a thermometer code over 0/255 slots
(so an ADE synapse 'matches' when its polarity agrees with the slot and a random
ADE fires iff its w synapses all match, rate 2^-w). Default is 52 slots over 9
blocks. NO long temporal window, per docs/state_window_gate.md: the only history
is a 12-tick ring feeding three rate/turn quantities.

Every knob env-configurable: _INPUT (width), _NCLASSES, _NADES, _WIDTHS
(clause widths), _FEATURES, _SPAN, _MODE, _DECAY_EVERY, _DECAY_SHIFT,
_MINOBS, _ADAPT_EVERY, _TARGET, _NETSEED, _NETLOG, _NET_RESET_ON_TARGET.

MEASURED SCALING (measure_bitbrain_scaling.nim, 3 recorded runs, 37412 ticks,
-d:release, one predict per power bin per tick, timed region = predicts only):
  RAM 1.59 MB default (98.6% SBC tensors); linear in nClasses, QUADRATIC in
      nAde, FLAT in input width; counted/bitset = 7.30x on RAM, ~1x on time.
  ms/tick 2.70 default = 21% of the 13.16 ms budget; 64 classes busts it (149%),
      nAde 512 uses 74%, nAde 64 uses 2%.
  CAPACITY vs ACCURACY: over a 100x RAM range the offline mean |err| moves
      17.254 -> 17.115 deg around Pattern's 16.964, and the sign flips along the
      nClasses axis, so it is noise, not a trend. The corrector is consistently
      slightly WORSE than Pattern. The ceiling is the STATE, not the classifier.
      VETO-CAPABLE OFFLINE CHECK ONLY (docs/offline_harness_trust.md), never
      presented as a live win.

ENGAGEMENT is proven, not assumed: test_bitbrain_net.nim (42 checks) shows 0
bytes before first use, different inputs -> different class outputs, a learn
raises SBC occupancy, bitset learn idempotent while counted learn is monotone,
threshold adaptation runs, and the global RNG is untouched.

Parity: shipped rack still onlyPattern, shipped movement still strafe. Guards:
test_env_report 25, test_rack_membership 48, test_tm_pattern_registration 20,
test_lead_gain_registration 13, test_lead_gain_legacy 24, test_bitbrain 56,
test_gun_harness 39, test_tfil_commit_env 30, test_bitbrain_net 42.
Clean archive build: [SuccessX].

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

common_libs/guns/lead_gain.nim

@@ -111,6 +111,7 @@ import std/[math, os, strutils, strformat, algorithm]
 import gun_harness/gun_interface
 import guns/tm_horizon
 import guns/pattern_matcher
+import gun_harness/selector
 
 const
   ## ── env knobs (all resolved once at gun construction) ─────────────────────
@@ -297,7 +298,10 @@ const
   ## Knobs that actually change behaviour (the rest are inert configuration kept
   ## for the boot report). A deprecation line is only worth printing for these
   ## plus the inert ones, because a stale inert name is still a stale name.
-  LegacyRackEnvName* = "TR_RACK_BITBRAIN"
+  ## The legacy rack knob, from `gun_harness/selector`'s alias table so the
+  ## two can never drift apart. It is the CURRENT name of rack id 17, which is
+  ## why it is not a distinct string.
+  LegacyRackEnvName* = RackLegacyAlias[0][0]
 
 proc netSwitchOn*(): bool =
   ## `TR_BITBRAIN_NET` unset/0 => the `TR_BITBRAIN_*` names are LEGACY aliases