debug: add fxcache key debug + fix PREFETCH_K hang

- Print fxcache lookup key to diagnose cache miss on H100
- PREFETCH_K=16 (was usize::MAX causing 8M PER samples in one shot)

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

crates/ml/examples/train_baseline_rl.rs

@@ -829,6 +829,7 @@ fn run_training(args: &Args) -> Result<Vec<RlTrainingResult>> {
         let key_hex = ml::feature_cache::calculate_dbn_cache_key_full(
             &symbol_dir, mbp10, trades,
         ).ok()?;
+        eprintln!("[DEBUG] fxcache key={}, looking in {:?}", key_hex, cache_dir);
         let key: [u8; 32] = hex::decode(&key_hex).ok()?.try_into().ok()?;
         let path = ml::fxcache::find_fxcache(&cache_dir, &key)?;
         match ml::fxcache::load_fxcache(&path) {