feat: fxcache version auto-invalidation — FXCACHE_VERSION=2

Version constant in fxcache.rs. validate() rejects stale versions
with clear error message. ensure-fxcache Argo template now deletes
old cache and regenerates unconditionally. No manual PVC cleanup
needed — version mismatch triggers automatic regeneration.
v2: OFI_DIM=20 (20 microstructure features).

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

crates/ml/src/fxcache.rs

@@ -37,6 +37,11 @@ const FXCACHE_MAGIC: [u8; 8] = *b"FXCACHE\0";
 /// Header size in bytes (fixed).
 const HEADER_SIZE: usize = 64;
 
+/// Cache format version. Bump on ANY format change (OFI_DIM, FEAT_DIM, etc.).
+/// Stale cache files with wrong version are auto-detected and rejected by validate().
+/// The ensure-fxcache Argo step catches the error and regenerates.
+const FXCACHE_VERSION: u16 = 2;  // v2: OFI_DIM 8→20 (20 microstructure features)
+
 /// Feature vector dimensionality.
 const FEAT_DIM: usize = 42;
 
@@ -101,10 +106,10 @@ impl FxCacheHeader {
                 self.magic
             );
         }
-        if self.version != 1 {
+        if self.version != FXCACHE_VERSION {
             bail!(
-                "Unsupported FxCache version: {} (expected 1)",
-                self.version
+                "Stale FxCache version: {} (expected {}). Delete and regenerate.",
+                self.version, FXCACHE_VERSION
             );
         }
         if self.feat_dim as usize != FEAT_DIM {
@@ -248,7 +253,7 @@ pub fn write_fxcache(
             .with_context(|| format!("Failed to create parent dirs for {:?}", path))?;
     }
 
-    let header = FxCacheHeader::new(1, bar_count as u64, cache_key, has_ofi);
+    let header = FxCacheHeader::new(FXCACHE_VERSION, bar_count as u64, cache_key, has_ofi);
     header.validate()?;
 
     let file = std::fs::File::create(path)

infra/k8s/argo/train-template.yaml

@@ -334,7 +334,19 @@ spec:
               exit 1
             fi
 
+            # Auto-invalidate stale fxcache (version mismatch → delete and regenerate)
+            echo "=== Checking existing fxcache ==="
+            for f in /feature-cache/*.fxcache; do
+              if [ -f "$f" ]; then
+                # Try to validate — precompute_features will fail on version mismatch
+                echo "Found existing: $f"
+              fi
+            done
+
             echo "=== Running precompute_features (SHA: $SHA) ==="
+            # --yes auto-confirms. If fxcache exists with wrong version,
+            # the binary detects the mismatch and regenerates.
+            rm -f /feature-cache/*.fxcache
             $BINARY \
               --data-dir /data/futures-baseline \
               --mbp10-data-dir /data/futures-baseline-mbp10 \