fix: fxcache to /workspace/ (PVC is read-only, workspace is emptyDir writable)

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

infra/k8s/argo/compile-and-train-template.yaml

@@ -63,7 +63,7 @@ spec:
       - name: trades-data-dir
         value: /data/futures-baseline-trades
       - name: feature-cache-dir
-        value: /data/feature-cache
+        value: /workspace/feature-cache
       - name: tx-cost-bps
         value: "0.1"
       - name: tick-size
@@ -302,11 +302,12 @@ spec:
               echo "ERROR: precompute_features not found in /workspace/bin/"
               exit 1
             fi
+            # Write to /workspace/ (writable emptyDir) — /data/ PVC is read-only
             $BINARY \
               --data-dir /data/futures-baseline \
               --mbp10-data-dir /data/futures-baseline-mbp10 \
               --trades-data-dir /data/futures-baseline-trades \
-              --output-dir /data/feature-cache \
+              --output-dir /workspace/feature-cache \
               --symbol ES.FUT \
               --yes
         volumeMounts: