cuda.core: a second grow of a grown VMM buffer fails

After one grow a buffer is backed by two physical chunks. The slow path retains only the handle of the mapping at buf.handle and maps aligned_prev_size bytes from it, but cuMemMap requires the size to equal the handle's full size, so the second grow fails. On main the attempt also unmaps the old range first and cannot restore it; with #2880 the old buffer stays intact and the grow raises cleanly. Fix belongs to the ownership redesign (epic #2906).

issue で説明されている cuda.core VMM バッファーの grow slow path から始め、buf.handle、aligned_prev_size、cuMemMap の呼び出しに注目してください。ownership redesign epic #2906 と #2880 の動作を確認してください。完了条件は、2 回目の grow が handle とサイズの不一致で失敗しなくなり、grow に失敗した場合も古いバッファーがそのまま保持されることです。