feat(phase-e-4-a): add mbp10_dir param to fxcache loader (signature-only)

Phase E.4.A Task 4: extend load_snapshots_from_fxcache with
`mbp10_dir: Option<&Path>`. When provided, the loader will peek
MBP-10 by timestamp and populate SnapshotRow.bid_l[1..10]/ask_l[1..10]
from real LOB depth — but the real-peek implementation lands in
Task 5 follow-on. This commit:
- introduces the parameter (callers pass None)
- warns at runtime if mbp10_dir Some until T5 lands
- enables downstream wiring of --use-real-depth + --mbp10-dir CLI
  flags in the smoke / backtest binaries

T5 deferred: on ES futures the --real-spread experiment showed 76%
of fxcache bars hit the 1-tick floor, so depth-from-MBP-10 likely
won't move the needle for ES. Higher-leverage work (Mamba2 wiring)
prioritised. T5 implementation reopens as a follow-on if E.4.A
gates pass with synthesised depth.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/ml/examples/alpha_compose_backtest.rs

@@ -382,6 +382,7 @@ fn main() -> Result<()> {
         cli.max_snapshots,
         Some(&alpha_cache),
         cli.real_spread,
+        None,  // E.4.A T4: mbp10_dir wiring + --use-real-depth flag lands in T5
     )?;
     let n_total = rows.len();
     info!("Loaded {} snapshots", n_total);

crates/ml/examples/alpha_dqn_h600_smoke.rs

@@ -650,6 +650,7 @@ fn main() -> Result<()> {
                 cli.max_snapshots,
                 alpha_cache_vec.as_deref(),
                 cli.real_spread,
+                None,  // E.4.A T4: mbp10_dir wiring + --use-real-depth flag lands in T5
             )?
         }
         (None, Some(mbp10)) => {

crates/ml/src/env/loaders.rs

@@ -127,7 +127,20 @@ pub fn load_snapshots_from_fxcache(
     max_snapshots: usize,
     alpha_cache: Option<&[f32]>,
     use_real_spread: bool,
+    // Phase E.4.A.4: when Some, peek MBP-10 by timestamp to populate
+    // SnapshotRow.bid_l[1..10] / ask_l[1..10] from real LOB depth.
+    // When None, synthesize L2-L10 at ±tick offsets (current behaviour).
+    // This commit introduces the parameter; the real-peek implementation
+    // lands in Task 5 follow-on.
+    mbp10_dir: Option<&Path>,
 ) -> Result<Vec<SnapshotRow>> {
+    if mbp10_dir.is_some() {
+        warn!(
+            "fxcache loader: mbp10_dir provided but real-peek not yet \
+             implemented (Phase E.4.A Task 5 follow-on) — falling back \
+             to synthesised L2-L10 offsets"
+        );
+    }
     let reader = FxCacheReader::open(fxcache_path)
         .with_context(|| format!("open fxcache {}", fxcache_path.display()))?;
     let alpha_dim = reader