feat: train-dqn template passes feature-cache-dir to trainer

Adds feature-cache-dir parameter (default: /data/feature-cache) so
the training binary can load pre-computed .fxcache files.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

infra/k8s/argo/train-dqn-template.yaml

@@ -52,6 +52,8 @@ spec:
         value: /data/futures-baseline-mbp10
       - name: trades-data-dir
         value: /data/futures-baseline-trades
+      - name: feature-cache-dir
+        value: /data/feature-cache
       - name: tx-cost-bps
         value: "0.1"
       - name: tick-size