Enable speculative decoding for Laguna (#2151)

The Laguna DFlash drafter was registered but unusable end to end. The
target exposed no rollback_speculative_cache and dropped capture_layer_ids,
so the dflash round loop refused to start; the drafter config also rejected
the published XS checkpoints over a missing sliding_windows field, which is
redundant with sliding_window and is now derived when absent.

Target validation now checks for the rollback hook so an incompatible target
fails at bind with a clear message.