BitBrain: TR_BITBRAIN_GAINS env knob (candidate set + fixed-gain degenerate)

Task A of campaign phase 2: the lead-gain candidate set is now pure env, so the
live arms need no recompile.

- common_libs/guns/bitbrain_gun.nim: BB_GAINS_ENV (TR_BITBRAIN_GAINS); the
  candidate list is parsed once at gun construction into a dynamic seq, so the
  hit counts/hit rates are sized to it. Unset/unparsable -> the shipped
  BB_CAND set [0,0.25,0.5,0.75,1.0] (byte-identical behaviour). Exactly ONE
  candidate degenerates to a FIXED gain applied from the first shot (learning
  bypassed), still gated to the long bands. parseGains clamps to [0,8],
  de-dupes and sorts so the argmax tie rule is unchanged. The [bb] line now
  prints the APPLIED gain AND the resulting angular shift, so a run's
  correction is auditable from stdout.
- ModularBot_garage/src/env_report.nim: emit TR_BITBRAIN_GAINS (resolved
  candidate set) and add BB_GAINS_ENV to the known-name list.
- tools/ab/arms_leadgain.txt: the 6-arm phase-2 sweep definition.

ModularBot_garage/src/env_report.nim

@@ -289,6 +289,8 @@ proc printEffectiveValues(ctx: EnvReportContext) =
   # `predict`, which is why `[rack]` membership is the actual enable switch.
   emit("TR_BITBRAIN_MEM", memModeName(ctx.bitbrain.memMode),
        sourceOf(BB_MEM_ENV))
+  emit("TR_BITBRAIN_GAINS", bbGainsString(ctx.bitbrain.cands),
+       sourceOf(BB_GAINS_ENV))
   emit("TR_BITBRAIN_N", $ctx.bitbrain.nClasses, sourceOf(BB_N_ENV))
   emit("TR_BITBRAIN_NADE", $ctx.bitbrain.nAde, sourceOf(BB_NADE_ENV))
   emit("TR_BITBRAIN_RANGE", $ctx.bitbrain.maxDeg, sourceOf(BB_RANGE_ENV))
@@ -371,7 +373,7 @@ proc knownEnvNames*(): seq[string] =
     TMH_SHIFT_ENV, TMH_BIG_MULT_ENV, TMH_LOG_ENV, TMH_RESET_ON_TARGET_ENV,
     TMH_WINDOW_ENV, TMH_RESET_DROP_ENV, TMH_NSTATES_ENV, TMH_ACCURVE_ENV,
     TMH_RETRAIN_EVERY_ENV, TMH_EPOCHS_ENV,
-    BB_MEM_ENV, BB_N_ENV, BB_NADE_ENV, BB_RANGE_ENV, BB_LOG_ENV,
+    BB_MEM_ENV, BB_GAINS_ENV, BB_N_ENV, BB_NADE_ENV, BB_RANGE_ENV, BB_LOG_ENV,
     BB_MIN_OBS_ENV, BB_WARMUP_ENV, BB_ADAPT_ENV, BB_CALIB_ENV, BB_DECAY_ENV,
     BB_DECAY_FRAC_ENV, BB_SEED_ENV, BB_RESET_ON_TARGET_ENV,
   ]