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).
f4ad96bacb