feat(ml-alpha): CfcConfig extended with n_batch + seq_len for v2 forward (X11a)

Adds CfcConfig.n_batch (default 1) and CfcConfig.seq_len (default 32).
PerceptionTrainer's trunk construction passes its training-time
n_batch/seq_len. Default values support backtester inference (B=1, K=32).

X11 foundation: subsequent commits add intermediate buffer fields +
forward_v2 method on CfcTrunk sized from these config fields. With X11
complete, fxt-backtest can drive the v2 forward through the trunk
without instantiating a full PerceptionTrainer.

Verification: ml-alpha + ml-backtesting + fxt-backtest all build clean.

Per spec §1.1 (X11 foundation).

crates/ml-alpha/src/cfc/trunk.rs

@@ -101,6 +101,11 @@ pub struct CfcConfig {
     /// (X1) and the per-stack Mamba2Block constructions added in X3/X5.
     /// Default matches `PerceptionTrainerConfig::default().mamba2_state_dim`.
     pub mamba2_state_dim: usize,
+    /// X11: inference batch + sequence dims for the v2 forward buffers
+    /// allocated on the trunk. Backtesters typically use (1, 32);
+    /// training-time consumers (PerceptionTrainer) pass their own dims.
+    pub n_batch: usize,
+    pub seq_len: usize,
 }
 
 impl Default for CfcConfig {
@@ -110,6 +115,8 @@ impl Default for CfcConfig {
             n_hid: HIDDEN_DIM,
             cfc_n_in: HIDDEN_DIM,
             mamba2_state_dim: 16,
+            n_batch: 1,
+            seq_len: 32,
         }
     }
 }

crates/ml-alpha/src/trainer/perception.rs

@@ -515,6 +515,8 @@ impl PerceptionTrainer {
                 n_hid: HIDDEN_DIM,
                 cfc_n_in: HIDDEN_DIM,
                 mamba2_state_dim: cfg.mamba2_state_dim,
+                n_batch: cfg.n_batch,
+                seq_len: cfg.seq_len,
             },
             cfg.seed,
         )