Clamp ragged batched speculative acceptance to fix phantom KV (#1962) (#2113)

Zeroing a rejected row's KV tail during batched speculative rollback left
those keys attended (q dot 0 still scores e^0 in the softmax denominator),
diluting rows that accepted fewer tokens. Clamp per-row acceptance to a
uniform count before rollback, and fail loud if a ragged batch ever reaches
a rectangular batch cache.