fix(architectural): include bar formation params in fxcache key + actually USE imbalance bars

## The bug (audit 2026-05-09)

`crates/ml/src/feature_cache.rs:calculate_dbn_cache_key_full` hashed only
`(symbol, data_source, dbn_filenames+sizes)` — NOT `imbalance_bar_threshold`
or `imbalance_bar_ewma_alpha`. Combined with `precompute_features.rs:346`
unconditionally calling `build_volume_bars` regardless of `data_source`,
this meant:

1. 14 audited production runs (Apr 11–May 8) all collided on the same
   fxcache key (`a3f933aa...` / `c07c960a...`) regardless of TOML
   `imbalance_bar_threshold` value
2. The imbalance-bar code path was reachable only via fxcache MISS, which
   never happens in production because `ensure-fxcache` always populates
   first
3. Every "tuning" of `imbalance_bar_threshold` across 16+ SP runs was a
   silent no-op — the system was actually running volume bars at
   DEFAULT_VOLUME_BAR_SIZE (100 contracts/bar)

## The fix (this commit)

**Part A — cache key includes bar formation params:**
- `calculate_dbn_cache_key_full` signature: 5 args → 7 args. Two new f64
  params hashed via `to_le_bytes()`.
- 4 callers updated atomically (per `feedback_no_partial_refactor`).
- 2 new unit tests (`test_cache_key_includes_bar_threshold`,
  `test_cache_key_includes_bar_alpha`) pin the contract.

**Part B — precompute_features actually USES data_source:**
- New CLI args `--imbalance-bar-threshold` (default 0.5) and
  `--imbalance-bar-ewma-alpha` (default 0.1) on both train_baseline_rl
  and precompute_features.
- `precompute_features.rs:346` now branches: when
  `data_source == "mbp10"` AND `mbp10_data_dir.is_some()`, calls
  `mbp10_to_imbalance_bars` instead of `build_volume_bars`.

**Argo plumbing:**
- `train-template.yaml` + `train-multi-seed-template.yaml`: new workflow
  parameters threaded into BOTH precompute and trainer invocations so
  both compute the same fxcache key.
- `scripts/argo-train.sh`: new CLI flags for ad-hoc overrides.
- ensure-fxcache regen path: removed `rm -f /feature-cache/*.fxcache`
  (with bar-params now in key, parallel experiments coexist).

## Effects going forward

- Tuning `imbalance_bar_threshold` actually changes bar density
- Configuring `data_source = "mbp10"` actually produces imbalance bars
- Multiple parallel experiments at different thresholds coexist on PVC
- `dqn-production.toml: imbalance_bar_threshold = 0.5` no longer ignored

Default values match prior production behavior → existing wgdc7-equivalent
runs reproduce, just with a *new* fxcache key (the old volume-bar cache
file is still on disk but won't be hit; harmless, can GC manually).

Audit-doc: `docs/dqn-wire-up-audit.md` updated with full context.
Tests: 5/5 feature_cache tests pass, full workspace + examples compile clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/argo-train.sh

@@ -31,6 +31,8 @@ FOLDS=1
 DRY_RUN=false
 TAG=""
 PROFILE=false
+IMBALANCE_BAR_THRESHOLD=""
+IMBALANCE_BAR_EWMA_ALPHA=""
 
 usage() {
   cat <<EOF
@@ -54,6 +56,11 @@ Options:
   --multi-seed <n>     Run N seeds in parallel (default: 1, fans out via DAG when >1)
   --folds <k>          Walk-forward fold count (default: 1, fans out via DAG when >1)
   --tag <t>            Label workflow with foxhunt-tag=<t> for log aggregation
+  --imbalance-bar-threshold <f>   Override imbalance bar threshold (default: 0.5)
+                                  MUST match between precompute_features and trainer.
+  --imbalance-bar-ewma-alpha <f>  Override imbalance bar EWMA alpha (default: 0.1).
+                                  α=1.0 disables adaptation (this codebase's reversed
+                                  convention). MUST match between precompute and trainer.
   --profile            Wrap training under nsys (NVIDIA Nsight Systems) and
                        upload .nsys-rep artefacts to MinIO bucket
                        foxhunt-training-artifacts/profiles/<sha>/. Forces the
@@ -91,6 +98,8 @@ while [[ $# -gt 0 ]]; do
     --folds)       FOLDS="$2"; shift 2 ;;
     --tag)         TAG="$2"; shift 2 ;;
     --profile)     PROFILE=true; shift ;;
+    --imbalance-bar-threshold)  IMBALANCE_BAR_THRESHOLD="$2"; shift 2 ;;
+    --imbalance-bar-ewma-alpha) IMBALANCE_BAR_EWMA_ALPHA="$2"; shift 2 ;;
     --dry-run)     DRY_RUN=true; shift ;;
     -h|--help)     usage ;;
     *)             echo "Unknown option: $1"; usage ;;
@@ -147,6 +156,8 @@ if [[ "$USE_MULTI_SEED" == "false" ]]; then
   [[ -n "$SYMBOL" ]]    && CMD="$CMD -p symbol=$SYMBOL"
   [[ -n "$CAPITAL" ]]   && CMD="$CMD -p initial-capital=$CAPITAL"
   [[ "$SANITIZER" != "none" ]] && CMD="$CMD -p sanitizer=$SANITIZER"
+  [[ -n "$IMBALANCE_BAR_THRESHOLD" ]]  && CMD="$CMD -p imbalance-bar-threshold=$IMBALANCE_BAR_THRESHOLD"
+  [[ -n "$IMBALANCE_BAR_EWMA_ALPHA" ]] && CMD="$CMD -p imbalance-bar-ewma-alpha=$IMBALANCE_BAR_EWMA_ALPHA"
 
   $BASELINE && CMD="$CMD -p hyperopt-trials=0"
   $WATCH    && CMD="$CMD --watch"
@@ -278,6 +289,8 @@ CMD="$CMD -p profile=$PROFILE"
 [[ -n "$SYMBOL" ]]   && CMD="$CMD -p symbol=$SYMBOL"
 [[ -n "$CAPITAL" ]]  && CMD="$CMD -p initial-capital=$CAPITAL"
 [[ -n "$TAG" ]]      && CMD="$CMD --labels foxhunt-tag=$TAG"
+[[ -n "$IMBALANCE_BAR_THRESHOLD" ]]  && CMD="$CMD -p imbalance-bar-threshold=$IMBALANCE_BAR_THRESHOLD"
+[[ -n "$IMBALANCE_BAR_EWMA_ALPHA" ]] && CMD="$CMD -p imbalance-bar-ewma-alpha=$IMBALANCE_BAR_EWMA_ALPHA"
 [[ "$SANITIZER" != "none" ]] && CMD="$CMD -p sanitizer=$SANITIZER"
 
 $BASELINE && CMD="$CMD -p hyperopt-trials=0"