Shorter Parakeet slices as Scriberr's memory fix

[2026-09-30] Shorter Parakeet slices as Scriberr's memory fix. I predicted ~6× less memory from the attention-matrix arithmetic. Measured, 300 → 120 s only went 9,384 → 6,510 MiB: a ~5.6 GB fixed floor dominates. expandable_segments:True was the real lever (5,496). Measure the process peak; never extrapolate it from one tensor.