fix(architectural): volume_bar_size in cache key + OFI front-month filter

## Two architectural cleanups, both surfaced by the wgdc8 experiment

### Part 1: volume_bar_size in cache key

Mirrors the imbalance_bar_threshold/ewma_alpha fix from `f7718b376`. The
volume bar size constant (100 contracts/bar) was previously hardcoded and
not in the fxcache key. Tuning it would have hit the same fossilization
bug as imbalance_bar_threshold did pre-fix.

Changes:
- `Hyperparams.volume_bar_size: u64` field added (default 100, matches
  `DEFAULT_VOLUME_BAR_SIZE` for backwards compat).
- TrainingProfile loader reads `volume_bar_size` TOML key.
- `calculate_dbn_cache_key_full` signature 7 → 8 args. Hashed via
  `to_le_bytes()`. Test `test_cache_key_includes_volume_bar_size`
  added; passes alongside the 5 existing tests.
- 4 callers updated atomically (per `feedback_no_partial_refactor`):
  `discover_and_load`, `data_loading.rs:146`, `train_baseline_rl.rs:599`,
  `precompute_features.rs:259,720`.
- `data_loading.rs:279` now passes `self.hyperparams.volume_bar_size`
  to `build_volume_bars` instead of the hardcoded `DEFAULT_VOLUME_BAR_SIZE`.
- New `--volume-bar-size` CLI arg on both binaries (default 100).
- New Argo workflow params `volume-bar-size` (default "100") and
  `data-source` (default "mbp10") on both `train-template.yaml` and
  `train-multi-seed-template.yaml`. Threaded into precompute + trainer
  invocations.
- `scripts/argo-train.sh` exposes `--volume-bar-size <n>` and
  `--data-source <s>` for ad-hoc overrides.

### Part 2: OFI front-month filter (latent bug fix)

`crates/ml/examples/precompute_features.rs:539-557` (the OFI/VPIN/Kyle's
Lambda computation branch when MBP-10 + trades data is available) was
loading trades unfiltered for per-bar microstructure feature computation.
The volume bar formation path filters front-month per-file (line 354), but
the OFI path did not.

Effect pre-fix: during contract rollover windows (e.g., ESZ24 → ESH25),
OFI per-bar microstructure features included trades from BOTH contracts
simultaneously, distorting VPIN, Kyle's Lambda, and trade imbalance
signals. Severity in production: small (front-month dominates ES.FUT
volume by 10-100×) but real and present in every prior MBP-10+trades
production run.

Fix: mirror the per-file `filter_front_month` call from the volume bar
path. Volume bar formation and OFI computation now both see the same
in-month trade tape. Added log line shows raw vs filtered count per file
for transparency.

## Why bundled

Both fixes touch trade-data plumbing in `precompute_features.rs` and the
fxcache key contract. Per `feedback_no_partial_refactor`, related
architectural cleanups land atomically. Both surfaced from the same
wgdc8 audit; bundling avoids two cache-key-invalidating commits in
sequence (each would force full fxcache regen).

## Compatibility

- `volume_bar_size` defaults to 100 → existing wgdc7-equivalent runs
  reproduce, but with a *new* fxcache key (the f7718b376-era cache file
  is unreachable; harmless, can GC manually).
- OFI fix is strictly more correct; no opt-out needed. Existing models
  trained on contaminated OFI features may show slight feature
  distribution drift on first cache regen — expected, not a regression.
- `data_source = "ohlcv"` Argo param now possible; routes precompute
  through volume bar branch directly. wgdc8 experiment uses this to test
  bar resolution sensitivity at volume_bar_size=500 (5× DEFAULT).

Tests: 6/6 feature_cache tests pass. Workspace + examples compile clean.
Audit-doc: `docs/dqn-wire-up-audit.md` updated.

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

scripts/argo-train.sh

@@ -33,6 +33,8 @@ TAG=""
 PROFILE=false
 IMBALANCE_BAR_THRESHOLD=""
 IMBALANCE_BAR_EWMA_ALPHA=""
+VOLUME_BAR_SIZE=""
+DATA_SOURCE=""
 
 usage() {
   cat <<EOF
@@ -61,6 +63,10 @@ Options:
   --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.
+  --volume-bar-size <n>           Override volume bar size in contracts (default: 100).
+                                  Used when --data-source != "mbp10". Cache key includes.
+  --data-source <s>               Data source mode: "mbp10" or "ohlcv" (default: "mbp10").
+                                  Threaded into both 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
@@ -100,6 +106,8 @@ while [[ $# -gt 0 ]]; do
     --profile)     PROFILE=true; shift ;;
     --imbalance-bar-threshold)  IMBALANCE_BAR_THRESHOLD="$2"; shift 2 ;;
     --imbalance-bar-ewma-alpha) IMBALANCE_BAR_EWMA_ALPHA="$2"; shift 2 ;;
+    --volume-bar-size)          VOLUME_BAR_SIZE="$2"; shift 2 ;;
+    --data-source)              DATA_SOURCE="$2"; shift 2 ;;
     --dry-run)     DRY_RUN=true; shift ;;
     -h|--help)     usage ;;
     *)             echo "Unknown option: $1"; usage ;;
@@ -158,6 +166,8 @@ if [[ "$USE_MULTI_SEED" == "false" ]]; then
   [[ "$SANITIZER" != "none" ]] && CMD="$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=$IMBALANCE_BAR_EWMA_ALPHA"
+  [[ -n "$VOLUME_BAR_SIZE" ]]          && CMD="$CMD -p volume-bar-size=$VOLUME_BAR_SIZE"
+  [[ -n "$DATA_SOURCE" ]]              && CMD="$CMD -p data-source=$DATA_SOURCE"
 
   $BASELINE && CMD="$CMD -p hyperopt-trials=0"
   $WATCH    && CMD="$CMD --watch"
@@ -291,6 +301,8 @@ CMD="$CMD -p profile=$PROFILE"
 [[ -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=$IMBALANCE_BAR_EWMA_ALPHA"
+[[ -n "$VOLUME_BAR_SIZE" ]]          && CMD="$CMD -p volume-bar-size=$VOLUME_BAR_SIZE"
+[[ -n "$DATA_SOURCE" ]]              && CMD="$CMD -p data-source=$DATA_SOURCE"
 [[ "$SANITIZER" != "none" ]] && CMD="$CMD -p sanitizer=$SANITIZER"
 
 $BASELINE && CMD="$CMD -p hyperopt-trials=0"