The first evaluated token attends to the positions above
pos - n_swa, so a SWA state starting at pos - n_swa + 1 holds the
whole window, but the reuse threshold asked for two more cells.
A saved SWA state keeps only the cells inside its window, so a
state read back from a slot file or from the RAM prompt cache was
always rejected and the slot rolled back to a checkpoint.
The SWA threshold now matches the window, while a state without SWA
keeps the exact bound. The test harness gains override_kv, used to
give the SWA test model a window smaller than the test context.
[no release]