fix: validate OFI data content, not just has_ofi flag — v2 cache had all-zero OFI

The v2 fxcache on PVC passed has_ofi=true validation (mbp10_dir was present
when built) but contained all-zero OFI data. The old binary set the flag
based on directory existence, not actual computed values.

Now counts non-zero OFI rows before accepting a cache — forces rebuild
when MBP-10 data is available but OFI content is all zeros.

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

crates/ml/examples/precompute_features.rs

@@ -217,12 +217,20 @@ async fn main() -> Result<()> {
     if early_check_path.exists() {
         match ml::fxcache::load_fxcache(&early_check_path) {
             Ok(cached) if cached.has_ofi || mbp10_dir.is_none() => {
-                let size_mb = std::fs::metadata(&early_check_path)
-                    .map(|m| m.len() as f64 / 1_048_576.0)
-                    .unwrap_or(0.0);
-                println!("Cache already exists: {} ({:.1} MB, has_ofi={}) — skipping extraction",
-                    early_check_path.display(), size_mb, cached.has_ofi);
-                return Ok(());
+                // has_ofi flag is necessary but not sufficient — verify actual OFI data is non-zero
+                let ofi_nonzero = cached.ofi.iter().filter(|r| r.iter().any(|&v| v != 0.0)).count();
+                if mbp10_dir.is_some() && ofi_nonzero == 0 {
+                    println!("Cache has_ofi=true but OFI data is all zeros ({} bars) — deleting stale cache",
+                        cached.ofi.len());
+                    std::fs::remove_file(&early_check_path).ok();
+                } else {
+                    let size_mb = std::fs::metadata(&early_check_path)
+                        .map(|m| m.len() as f64 / 1_048_576.0)
+                        .unwrap_or(0.0);
+                    println!("Cache already exists: {} ({:.1} MB, has_ofi={}, ofi_nonzero={}) — skipping extraction",
+                        early_check_path.display(), size_mb, cached.has_ofi, ofi_nonzero);
+                    return Ok(());
+                }
             }
             Ok(_) => {
                 println!("Cache exists but has_ofi=false while MBP-10 data available — deleting stale cache");