feat(foxhuntq): Phase 1c snapshot-resolution alpha + leakage fix + variable-dim fxcache

Three things landing atomically because they're load-bearing for each other:

1. **Trend-scanning leakage fix** — trend_scanning.rs was emitting OLS slope+t-stat
   over a *forward* window [t, t+L]. With the Phase 1a label = sign(price[t+60]
   − price[t]), the forward feature window overlaps the label window, contaminating
   it. Purged walk-forward only sterilizes forward-looking *labels* that cross
   the train/val split, not forward-looking *features* that peek inside the same
   horizon the label measures. The leak inflated MLP accuracy from 0.49
   (legacy 74-dim baseline) to 0.75 — vanished to 0.50 after switching to a
   trailing window. Bounded the perfect-fit t-stat sentinel from ±1e6 → ±20
   (p<1e-30 is already meaningless); eliminated the 16k corruption-cap drops.

2. **Variable-dim alpha column** — fxcache schema now carries the alpha-feature
   width via metadata (`alpha_feature_dim`), not a compile-time constant. Same
   on-disk format hosts the 134-dim bar-level stack OR the 81-dim snapshot stack.
   Reader + auto-detect honor the metadata-declared dim; downstream MLP auto-sizes
   `in_dim`. Single schema, no forks.

3. **Snapshot pipeline (Phase 1c falsification)** — `snapshot_pipeline.rs`: 81-dim
   per-MBP10-snapshot extractor reusing 10 snapshot-native alpha blocks + 6 new
   snapshot-specific features (time-since-trade, time-since-snap, event-rate,
   spread-bps, L1-imbalance, microprice-mid drift). `precompute_features` gets
   `--row-unit snapshot` flag; emits one fxcache row per LOB update (1.97M rows
   from MBP-10 data vs 206K for bar mode).

**Smoke verdict on real data** (ES.FUT, 1.97M snapshots, 384K val):
- Bar-level honest alpha: accuracy=0.5005, AUC=0.5043 (no signal)
- **Snapshot-level alpha**: accuracy=0.5241, AUC=0.6849 (real signal, 384K val)
- GBM corroboration: accuracy=0.5401 (non-linear partitioning sees more)
- Horizon decay: alpha peaks at K=20-50 snapshots (~5-25ms), gone by K=500
- Regime-conditional: spread-Q4 quintile hits 0.752 accuracy on 76k samples

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

crates/ml/Cargo.toml

@@ -212,7 +212,7 @@ num-traits = "0.2"
 # Parquet I/O for feature caching (Wave 2 Agent 8)
 # Updated to workspace version 56 to fix arrow-arith compilation conflict
 parquet.workspace = true
-arrow.workspace = true
+arrow = { workspace = true, features = ["ipc"] }  # ipc feature: fxcache uses Arrow IPC format
 bytes = "1.5"  # For Parquet in-memory serialization
 num = "0.4"
 libc = "0.2"