Attention is the expensive part at video token counts — ~37k tokens for 93
frames at 480x832, and it is O(N^2) in that. The checkpoint already carries
bsa_params with sparsity 0.9375, roughly a 16x cut, and upstream's
implementation is pure Triton, so none of it needs the flash-attn source build.

So it is now a setting: longcat.bsa server-wide, "bsa" per model, --bsa on the
service. Off by default because it is an approximation and video shows
approximation as temporal flicker, not one soft frame. "auto" stays off when
triton is missing; "on" says so instead, because someone who asked for it would
otherwise be left wondering why nothing got faster. BSA replaces the dense
backend rather than joining it, since Attention tests enable_bsa first.

And cp_split_hw, found while wiring this: the DiT's forward opens with
`if self.cp_split_hw[0] * self.cp_split_hw[1] > 1:` and has no None guard, but
dit/config.json ships "cp_split_hw": null — so a DiT built from the config as
published raises TypeError on its first forward. Our only assignment made it
worse by storing cp_size, a process count, where a [h, w] pair belongs: an int
is not subscriptable either. It defaults to [1, 1] now, derives [1, cp_size]
under context parallelism, and takes an explicit tile that is checked against
the group size.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0185zEBiy3KWB37xNGSrxL7w