Loader can now serve sequences in temporal order (Sequential mode);
when active, the trainer skips attn_pool reset at non-file-boundary
sequence boundaries (next commit will use this for stateful CfC + MTER).
Random mode preserved as default; ZERO behavioral change for existing
runs. Phased commit 1/8 of intervention B per
docs/superpowers/specs/2026-05-21-crt-train-intervention-b-multi-timescale-readout.md
7dd953aa2b